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Homo sapiens and early human migration

Homo sapiens, the first modern humans, evolved from their early hominid predecessors between 200,000 and 300,000 years ago. They developed a capacity for language about 50,000 years ago. The first modern humans began moving outside of Africa starting about 70,000-100,000 years ago. Humans are the only known species to have successfully populated, adapted to, and significantly altered a wide variety of land regions across the world, resulting in profound historical and environmental impacts.

Where do we begin?

Before we tell the stories that make up world history, it is useful to ask: where do we begin? Where did our human stories start? Homo sapiens is part of a group called hominids, which were the earliest humanlike creatures. Based on archaeological and anthropological evidence,

We think that hominids diverged from other primates somewhere between 2.5 and 4 million years ago in eastern and southern Africa. Though there was a degree of diversity among the hominid family, they all shared the trait of bipedalism, or the ability to walk upright on two legs.

Evolution

Scientists have several theories about why early hominids evolved. One, the aridity hypothesis, suggests that early hominids were more suited to dry climates and evolved as the Africa’s dry savannah regions expanded.

According to the savannah hypothesis, early tree-dwelling hominids may have been pushed out of their homes as environmental changes caused the forest regions to shrink and the size of the savannah expand. These changes, according to the savannah hypothesis, may have caused them to adapt to living on the ground and walking upright instead of climbing. 2^22squared

Hominids continued to evolve and develop unique characteristics. Their brain capacities increased, and approximately 2.3 million years ago, a hominid known as Homo habilis began to make and use simple tools. By a million years ago, some hominid species, particularly Homo erectus, began to migrate out of Africa and into Eurasia, where they began to make other advances like controlling fire

Though there were once many kinds of hominids, only one remains: Homo sapiens. Extinction is a normal part of evolution, and scientists continue to theorize why other hominid species didn’t survive. We do have some clues as to why some species were less successful at surviving than others, such as an inability to cope with competition for food, changes in climate, and volcanic eruptions.

Migration and the Peopling of the Earth

How and why?

Between 70,000 and 100,000 years ago, Homo sapiens began migrating from the African continent and populating parts of Europe and Asia. They reached the Australian continent in canoes sometime between 35,000 and 65,000 years ago.

Scientists studying land masses and climate know that the Pleistocene Ice Age created a land bridge that connected Asia and North America (Alaska) over 13,000 years ago. A widely accepted migration theory is that people crossed this land bridge and eventually migrated into North and South America

How were our ancestors able to achieve this feat, and why did they make the decision to leave their homes? The development of language around 50,000 years ago allowed people to make plans, solve problems, and organize effectively. We can’t be sure of the exact reasons humans first migrated off of the African continent, but it was likely correlated with a depletion of resources (like food)

in their regions and competition for those resources. Once humans were able to communicate these concerns and make plans, they could assess together whether the pressures in their current home outweighed the risk of leaving to find a new one.

Adaptation and effects on nature

When humans migrated from Africa to colder climates, they made clothing out of animal skins and constructed fires to keep themselves warm; often, they burned fires continuously through the winter. Sophisticated weapons, such as spears and bows and arrows, allowed them to kill large mammals efficiently.

Along withchangingclimates,these hunting methods contributed to the extinction of giant land mammals such as mammoths, giant kangaroos, and mastodons. Fewer giant mammals, in turn, limited hunters’ available prey.

In addition to hunting animals and killing them out of self-defense, humans began to use the earth’s resources in new ways when they constructed semi-permanent settlements. Humans started shifting from nomadic lifestyles to fixed homes, using the natural resources there

Semi-permanent settlements would be the building-blocks of established communities and the development of agricultural practices.

The history of humanity in your face

The face you see in the mirror is the result of millions of years of evolution and reflects the most distinctive features that we use to identify and recognize each other, molded by our need to eat, breath, see, and communicate.

But how did the modern human face evolve to look the way it does? Eight of the top experts on the evolution of the human face, including Arizona State University's William Kimbel, collaborated on an article published this week in the journal Nature Ecology & Evolution to tell this four-million-year story. Kimbel is the director of the Institute of Human Origins and Virginia M. Ullman Professor of Natural History and the Environment in the School of Human Evolution and Social Change.

After our ancestors stood on two legs and began to walk upright, at least 4.5 million years ago, the skeletal framework of a bipedal creature was pretty well formed. Limbs and digits became longer or shorter, but the functional architecture of bipedal locomotion had developed.

But the skull and teeth provide a rich library of changes that we can track over time, describing the history of evolution of our species. Prime factors in the changing structure of the face include a growing brain and adaptations to respiratory and energy demands, but most importantly, changes in the jaw, teeth, and face responded to shifts in diet and feeding behavior. We are, or we evolved to be, what we eat -- literally!

Diet has played a large role in explaining evolutionary changes in facial shape. The earliest human ancestors ate tough plant foods that required large jaw muscles and cheek teeth to break down, and their faces were correspondingly broad and deep, with massive muscle attachment areas.

As the environment changed to drier, less wooded conditions, especially in the last two million years, early Homo species began to routinely use tools to break down foods or cut meat. The jaws and teeth changed to meet a less demanding food source, and the face became more delicate, with a flatter countenance.

Changes in the human face may not be due only to purely mechanical factors. The human face, after all, plays an important role in social interaction, emotion, and communication. Some of these changes may be driven, in part, by social context. Our ancestors were challenged by the environment and increasingly impacted by culture and social factors. Over time, the ability to form diverse facial expressions likely enhanced nonverbal communication.

Large, protruding brow ridges are typical of some extinct species of our own genus, Homo, like Homo erectus and the Neanderthals. What function did these structures play in adaptive changes in the face? The African great apes also have strong brow ridges, which researchers suggest help to communicate dominance or aggression

It is probably safe to conclude that similar social functions influenced the facial form of our ancestors and extinct relatives. Along with large, sharp canine teeth, large brow ridges were lost along the evolutionary road to our own species, perhaps as we evolved to become less aggressive and more cooperative in social contexts.

"We are a product of our past," says Kimbel. "Understanding the process by which we became human entitles us to look at our own anatomy with wonder and to ask what different parts of our anatomy tell us about the historical pathway to modernity."

seven million years ago

On the biggest steps in early human evolution scientists are in agreement. The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs. They were flaking crude stone tools by 2.5 million years ago

By the time our planet was four billion years old, the rise of large plants and animals was just beginning. Complexity exploded around that time, as the combination of multicellularity, sexual reproduction, and other genetic advances brought about the Cambrian explosion. Many evolutionary changes occurred over the next 500 million years, with extinction events and selection pressures paving the way for new forms of life to arise and develop.

65 million years ago, a catastrophic asteroid strike wiped out not only the dinosaurs, but practically every animal weighing over 25 kg (excepting leatherback sea turtles and some crocodiles). This was Earth's most recent great mass extinction, and left a large number of niches unfilled in its wake. Mammals rose to prominence in the aftermath, with the first humans arising less than 1 million years ago. Here's our story.

65 million years ago, a massive asteroid somewhere between 5 and 10 kilometers in diameter struck our planet. It kicked up a layer of dust that settled all over the world, a layer that can be found today in our planet's sedimentary rock. On the older side of that layer, fossils such as dinosaurs, pterosaurs, ichthyosaurs and plesiosaurs are abundant. Giant reptiles, ammonites, and large classes of plants and animals all existed prior to that event, along with small, flying birds and the tiny, land-dwelling mammals.

After that event, the mammals survived. With no larger predators to stop them, they grew, diversified, and experienced a population explosion. Primates, rodents, lagomorphs, and other forms of mammals, including placental mammals, marsupials, and even the egg-laying mammals are all abundant at the start of the Cenezoic epoch.

Almost immediately, the primates began diversifying even further. 63 million years ago — just 2 million years after the demise of the dinosaurs — they split into two groups.

The dry-nosed primates, known formally as the haplorrhines, which developed into modern monkeys and apes. The wet-nosed primates, known as the strepsirrhines, which developed into modern lemurs and aye-ayes.

58 million years ago, another big change occurred: the haplorrhines experienced an interesting genetic split, as the first novel and unique evolutionary branch became distinct from the rest of the dry-nosed primates: the tarsier. With its enormous eyes, it was uniquely well-adapted to see at night.

The niche it now occupied was sufficiently different from the remaining groups of our ancestors that they evolved differently from the rest of their cousins from this point onwards. This type of evolutionary splitting occurs every so often, and isn't unique to primates.

Although we normally don't think very much about our distant cousins and how they develop once they've split off from us, it isn't just haplorrhines like us (and our direct ancestors) that underwent interesting phases of evolution. All throughout the past 65 million years — just as it was before that time — the various mammals, birds, plants, and other living organisms evolved together. Evolution is driven by environmental changes, and that includes all the floral and faunal changes that occur on our planet.

55 million years ago, a sudden rise in greenhouse gases causes the global average temperature to swiftly rise, wiping out many deep-ocean animals and plants. This transformation left many large, unfilled niches in the ocean, paving the way for cetaceans (the large oceanic mammals) to develop.

50 million years ago, some of the even-toed mammals begin evolving into sea-dwelling creatures. The artiodactyls may have all evolved from a single, common ancestor, or may have evolved independently. Animals like Indohyus, which dates to 48 million years ago, may have given rise to protocetids: shallow-water mammals that returned to land to give birth

Right around that time, 47 million years ago, the primate Darwinius masillae existed, as the fossil Ida, preserved from that time, provides a spectacular example. Although this was originally touted as a proverbial "missing link" in human evolution, Ida is not a haplorrhine like us, but a strepsirrhene: a wet-nosed primate.

But another 7 million years later — 40 million years ago — an important development occurred among the dry-nosed primates: the New World monkeys branched off. Humans and our ape ancestors are descended from Old World monkeys; New World monkeys are the first simians (or higher primates) to evolutionary diverge from our lineage. They would go on to colonize most of South America, where they are still found in abundance

The Old World monkeys continue to thrive and successfully occupy their niches, while diversifying in body size and physical features. 25 million years ago, the first apes arise, splitting off from the remaining Old World monkeys at this time. The apes — defined by the complete lack of a tail of any type — would go on to give rise to many of the close relatives of humans that survive today: both the lesser apes and the great apes

The earliest ape to split off from the Old World monkeys was the Gibbon, a lesser ape that first arose 18 million years ago.

Sometime between 14 and 16 million years ago, the first great apes arose, with Orang-utans branching off 14 million years ago. The Orang-utans spread into southern Asia after this, while the other great apes remained in Africa. The largest primate ever, Gigantopithecus, first arose some 9 million years ago, only becoming extinct a few hundred thousand years ago.

7 million years ago, gorillas branched off from the other great apes; they remain the largest of all the surviving primates.

The great apes split off in two directions 6 million years ago, with one direction giving rise to humanity's ancestors and the other branch giving rise to chimpanzees and bonobos. The chimpanzee/bonobo branch remains unified for another 4 million years, with our closest surviving relatives — the chimpanzees and bonobos — diverging from one another a mere 2 million years ago.

But along the track of our direct ancestors, the developments were rapid and profound. 5.6 million years ago, the first truly bipedal ape, Ardipithecus, arose. Although it's a controversial claim, the hand bones in Ardipithecus show evidence of it being a transitional fossil between the earlier great apes and the later Australopithecines.

Approximately 4 million years ago, the first Australopithecus evolved: the first members of the Hominina subtribe (a taxonomic classification more specific than family but less specific than genus). Shortly thereafter, the first evidence of stone tool use appears: presently at 3.4-to-3.7 million years ago.

A critical evolutionary step happened a little more than 2 million years ago, as our hominid ancestors faced food shortages. One evolutionarily successful approach was to develop stronger jaws, which gave us the capability to eat foods (like nuts) that were otherwise inaccessible. But another approach was also successful: to develop weaker jaws and larger brains, enabling us to access the food.

While both groups survived for a time, the larger-brained group was more adaptable to changes, and they continued to survive. This is the evolutionary path that we think led to the development of the genus Homo, which first arose about 2.5 million years ago.

Homo habilis, known colloquially as "handy man," had larger brains than their Australopithecus counterparts and displayed far more widespread tool use. The group of hominids shown here includes many of our direct ancestors and evolutionary cousins....

Shown here are Homo sapiens (modern humans), Australopithecus afarensis (thought to be the direct ancestor of the genus Homo), Homo erectus (which arose 1.9 million years ago and only died out ~140,000 years ago), Homo habilis (the first member of the genus Homo), and the Neanderthal (which arose later than, and independently of, modern humans). (Photo by: Encyclopaedia Britannica/UIG via Getty Images)

About 1.9 million years ago, Homo erectus evolved. This human ancestor not only walked fully upright, but had much larger brains than Homo habilis: nearly twice as large, on average. Homo erectus became the first direct human ancestor to leave Africa, and the first to display evidence of using fire. Homo habilis was likely driven to extinction more than a million years ago, as was the last Australopithecus.

Across the world, new examples of the genus Homo emerged, including Homo antecessor in Europe (which may be an evolved habilis or erectus, or an early form of heidelbergensis) about 1.2 million years ago, followed by Homo heidelbergensis some 600,000 years ago. Approximately 700,000 years ago, the earliest evidence for cooking appears; about 500,000 years ago, the first evidence for clothing appears.

Approximately 300,000 years ago, the first Homo sapiens — anatomically modern humans — arose alongside our other hominid relatives. It is unknown whether we descended directly from Homo erectus, heidelbergensis, or antecessor, although Neanderthals, which came slightly later at 240,000 years ago, most certainly came from Homo heidelbergensis. Modern speech is thought to have arisen almost as soon as Homo sapiens did.

It took 13.8 billion years of cosmic history for the first human beings to arise, and we did so relatively recently: just 300,000 years ago. 99.998% of the time that passed since the Big Bang had no human beings at all; our entire species has only existed for the most recent 0.002% of the Universe. Yet, in that short time, we've managed to figure out the entire cosmic story that led to our existence. Fortunately, the story won't end with us, as it's still being written.

What Was It Like When The Universe Was Inflating?

Our Universe today is full of matter and radiation, and can be observed by us through a variety of means. Atoms have clumped and clustered together due to billions of years of gravitation.

This has formed a great cosmic web on the largest scales, with clusters of galaxies, individual galaxies, clouds of gas, stars, planets, and more on smaller scales. Through it all, the Universe has been expanding and cooling, something it's been doing since the earliest moments of the hot Big Bang.

But the Big Bang wasn't the very beginning of the Universe. Before that, there was a period known as cosmic inflation, which came earlier and set up the hot Big Bang. While living in an expanding, cooling Universe is difficult to intuit, inflation paints an entirely different picture. Here's what it would be like to live in an inflating Universe.

Imagine that you were a particle, located somewhere in the fabric of spacetime. A short distance away, another particle also exists. Imagine that the only thing that impacts them is the expansion of the Universe. How, then, will this particle move relative to you?

If your Universe were filled with radiation, it would expand like the square root of time: the distance between you and this particle scales as ~t1/2. If your Universe were filled with matter, it would expand like time to the two-thirds power: the distance between you and this particle scales as ~t2/3.

This means that after a certain amount of time, this particle would double its distance from you. Because inflation is not only exponential but also rapid — the expansion rate is very large during inflation — that doubling only requires somewhere in the neighborhood of 10-35 seconds

But the defining trait of inflation isn't its rapidity, since, after all, the early stages of the hot Big Bang may be just as rapid. Instead, the defining trait of inflation is its relentlessness.

And we can continue this as long as we want. After 10-34 seconds of inflation, the nearby particle would be 1024 times as far away as it was initially. After 10-33 seconds, it would be 1030 times as far as its initial distance.

And after 10-30 seconds of inflation, this particle would be about 1030000 times as distant as it was initially. If your Universe began full of particles of any type, they would in extraordinarily short order be driven away from one another so that no two ever saw each other again.

Space itself may have begun with an interesting intrinsic curvature to it. It could have been balled-up, knotted, twisted-and-turned, or even spherical. It could have been full of topological defects, with holes throughout it. It could have been connected in multiple places in bizarre ways. It could have even contained the entirety of space within a volume as minuscule as a subatomic particle.

But during inflation, this rapid-and-relentless expansion will increase the size of the Universe many, many times over: by the same amount that it would push any other particle away. It will take any initial geometry and stretch it to such a large scale that any region you look at — even something as large as our entire observable Universe today — would be indistinguishable from spatially flat.

The reason inflation works this way is because there's a large amount of energy that's intrinsic to space itself. As the fabric of the Universe expands, new space gets created, also with that same amount of energy inherent to it. This is why the expansion is relentless. If you look at an inflating Universe, it continues to inflate in an ongoing fashion, never decreasing in its rapidity.

But on the very smallest scales, under these conditions, there are also quantum fluctuations occurring.

These fluctuations happen in our Universe today, only they occur both on very low energy scales and on timescales that are extremely short compared to anything we observe. If you visualize these fluctuations as virtual particle-antiparticle pairs popping in-and-out of existence, they do so on timescales that are far too short to result in anything interesting happening; they simply add a small amount of extra energy to the fabric of space itself.

Every 10-33 to 10-32 seconds, the smallest subatomic scale we can describe with our physical laws known today — the Planck scale — gets stretched to the size of our presently observable Universe. On longer timescales than that, what was previously created would then become unobservable.

Inflation, remember, is relentless, and what happened just a tiny fraction of a second ago is now more than an entire visible Universe away. On all scales, from the very small to the very large, there should be these quantum fluctuations not only imprinted, but continuously newly imprinting on the Universe.

Yet inflation doesn't last forever everywhere in the Universe. Every time new space is created, there's a small-but-finite probability that inflation will be brought closer to its inevitable end. One way to visualize whether inflation ends or not is to picture a ball that rolls very, very slowly atop a plateau. Below the plateau is a valley that lies below; if the ball rolls into the valley, inflation ends.

When you create new space, there's again a random distribution of probabilities: whether the ball rolls closer to the plateau's center or closer to the edge. For the places where the ball reaches the edge and rolls into the valley, inflation ends and the energy transforms into the energy of the hot Big Bang.

It was very likely that the first regions to undergo this transition weren't the ones that became our observable Universe, but that we survived while these other Big Bangs occurred elsewhere in our inflating Universe. Most of them were incredibly distant, but some of them may have occurred very close to the region that eventually became our Universe.

As long as inflation goes on, space continues to be filled with these energy fluctuations on all scales, creating a fabric of space that appears like a continuously vibrating grid. Not just on one scale, like we imagine a passing gravitational wave would induce, but on all scales.

Finally, inflation comes to an end where we are. It's as though all of this energy inherent to space, with slightly different values at different locations, all comes tumbling down. It transforms into matter, antimatter, and radiation, and creates a Universe that is now hot, dense, and uniform in temperature, rather than cold and empty.

This transition is known as cosmic reheating, and it marks the transition from an inflationary spacetime into the beginning of our hot Big Bang. The energy fluctuations become density fluctuations, which gives rise to the large-scale structure in our Universe today.

When inflation comes to an end, our Universe as-we-know-it begins.

In theory, what lies beyond the observable Universe will forever remain unobservable to us, but there are very likely large regions of space that are still inflating even today. Once your Universe begins inflating, it's very difficult to get it to stop everywhere. For every location where it comes to an end, there's a new, equal-or-larger-sized location getting created as the inflating regions continue to grow.

Even though most regions will see inflation end after just a tiny fraction of a second, there's enough new space getting created that inflation should be eternal to the future.

Inflation set up and created the entire observable Universe, and gave the hot Big Bang the conditions we need it to have to be consistent with what we observe. But the inflationary Universe was dramatically different than the Universe we observe today. In order to understand and visualize it, we have to put our intuition aside, and embrace a reality where the only energy that matters is the energy intrinsic to the fabric of space itself.

What Was It Like When The Big Bang First Began?

Looking out at our Universe today, we not only see a huge variety of stars and galaxies both nearby and far away, we also see a curious relationship: the farther away a distant galaxy is, the faster it appears to move away from us. In cosmic terms, the Universe is expanding, with all the galaxies and clusters of galaxies getting more distant from one another over time. In the past, therefore, the Universe was hotter, denser, and everything in it was closer together.

If we extrapolate back as far as possible, we'd come to a time before the first galaxies formed; before the first stars ignited; before neutral atoms or atomic nuclei or even stable matter could exist. The earliest moment at which we can describe our Universe at hot, dense, and uniformly full-of-stuff is known as the Big Bang. Here's how it first began.

Some of you are going to read that last sentence and be confused. You might ask, "isn't the Big Bang the birth of time and space?" Sure; that's how it was originally conceived. Take something that's expanding and of a certain size and age today, and you can go back to a time where it was arbitrarily small and dense. When you get down to a single point, you'll create a singularity: the birth of space and time.

Only, there's a ton of evidence that points to a non-singular origin to our Universe. We never achieved those arbitrarily high temperatures; there's a cutoff. Instead, our Universe is best described by an inflationary period that occurred prior to the Big Bang, and the Big Bang is the aftermath of what occurred at the end of inflation. Let's walk through what that looked like.

During inflation, the Universe is completely empty. There are no particles, no matter, no photons; just empty space itself. That empty space has a huge amount of energy in it, with the exact amount of energy slightly fluctuating over time.

Those fluctuations get stretched to larger scales, while new, small-scale fluctuations are created on top of that. (We described what the Universe looked like during inflation previously.)

This continues as long as inflation goes on. But inflation will come to an end randomly, and not in all locations at once. In fact, if you lived in an inflating Universe, you'd likely experience a nearby region have inflation come to an end, while the space between you and it expanded exponentially. For a brief instant, you'd see what happens at the start of a Big Bang before that region disappeared from view.

In an initially, relatively small region, perhaps no bigger than a soccer ball but perhaps much larger, the energy inherent to space gets converted into matter and radiation. The conversion process is relatively fast, taking approximately 10-33 seconds or so, but not instantaneous. As the energy bound up in space itself gets converted into particles, antiparticles, photons and more, the temperature starts to rapidly rise.

Because the amount of energy that gets converted is so large, everything will be moving close to the speed of light. They will all behave as radiation, whether the particles are massless or massive doesn't matter. This conversion process is known as reheating, and signifies when inflation comes to an end and the stage known as the hot Big Bang begins.

In terms of the expansion speed, you'll witness a tremendous change. In an inflationary Universe, space expands exponentially, with more distant regions accelerating away as time goes on. But when inflation ends, the Universe reheats, and the hot Big Bang starts, more distant regions will recede from you more slowly as time goes on.

From an outside perspective, the part of the Universe where inflation ends sees the expansion rate there drop, while the inflating regions surrounding it see no such drop.

Probability-wise, it's extremely likely that from the perspective of whatever region of inflating space you're in prior to the Big Bang, you'll see inflation end in nearby regions many times. These locations where inflation ends will quickly fill with matter, antimatter, and radiation, and expand more slowly than the still-inflating regions do.

These regions will expand away from all the other locations where inflation still goes on exponentially, meaning they will very quickly recede from view. In the standard inflationary picture, because of this expansion rate change, there's virtually no chance that any two Universes, where separate hot Big Bangs occur, will ever collide or interact.

Finally, the region where we will come to live gets cosmically lucky, and inflation comes to an end for us. The energy that was inherent to space itself gets converted to a hot, dense, and almost uniform sea of particles.

The only imperfections, and the only departures from uniformity, correspond to the quantum fluctuations that existed (and were stretched across the Universe) during inflation. The positive fluctuations correspond to initially overdense regions, while the negative fluctuations get converted into initially underdense regions.

We cannot observe these density fluctuations, today, as they were when the Universe first underwent the hot Big Bang. There are no visual signatures we can access from that early on; the first one we've ever accessed come from 380,000 years later, after they've undergone countless interactions.

Even at that, we can extrapolate back what the initial density fluctuations were, and find something extremely consistent with the story of cosmic inflation. The temperature fluctuations that are imprinted on the first picture of the Universe — the cosmic microwave background — gives us confirmation of how the Big Bang began.

What might be observable to us, however, are the gravitational waves left over from the end of inflation and the start of the hot Big Bang. The gravitational waves that inflation generates move at the speed of light in all directions, but unlike the visual signatures, no interactions can slow them down.

They will arrive continuously, from all directions, passing through our bodies and our detectors. All we need to do, if we want to understand how our Universe got its start, is find a way to observe these waves either directly or indirectly. While many ideas and experiments abound, none have returned a successful detection so far.

Once inflation comes to an end, and all the energy that was inherent to space itself gets converted into particles, antiparticles, photons, etc., all the Universe can do is expand and cool.

Everything smashes into one another, sometimes creating new particle/antiparticle pairs, sometimes annihilating pairs back into photons or other particles, but always dropping in energy as the Universe expands.

The Universe never reaches infinitely high temperatures or densities, but still attains energies that are perhaps a trillion times greater than anything the LHC can ever produce.

The tiny seed overdensities and underdensities will eventually grow into the cosmic web of stars and galaxies that exist today. 13.8 billion years ago, the Universe as-we-know-it had its beginning. The rest is our cosmic history.

What Was It Like When Life's Complexity Exploded?

The Universe was already two-thirds of its present age by the time the Earth formed, with life emerging on our surface shortly thereafter. But for billions of years, life remained in a relatively primitive state. It took nearly a full four billion years before the Cambrian explosion came: where macroscopic, multicellular, complex organisms — including animals, plants, and fungi — became the dominant lifeforms on Earth.

As surprising as it may seem, there were really only a handful of critical developments that were necessary in order to go from single-celled, simple life to the extraordinarily diverse sets of creatures we'd recognize today.

We do not know if this path is one that's easy or hard among planets where life arises. We do not know whether complex life is common or rare. But we do know that it happened on Earth. Here's how.

Once the first living organisms arose, our planet was filled with organisms harvesting energy and resources from the environment, metabolizing them to grow, adapt, reproduce, and respond to external stimuli.

As the environment changed due to resource scarcity, competition, climate change and many other factors, certain traits increased the odds of survival, while other traits decreased them. Owing to the phenomenon of natural selection, the organisms most adaptable to change survived and thrived.

Relying on random mutations alone, and passing those traits onto offspring, is extremely limiting as far as evolution goes. If mutating your genetic material and passing it onto your offspring is the only mechanism you have for evolution, you might not ever achieve complexity.

But many billions of years ago, life developed the ability to engage in horizontal gene transfer, where genetic material can move from one organism to another via mechanisms other than asexual reproduction.

Transformation, transduction, and conjugation are all mechanisms for horizontal gene transfer, but they all have something in common: single-celled, primitive organisms that develop a genetic sequence that's useful for a particular purpose can transfer that sequence into other organisms, granting them the abilities that they worked so hard to evolve for themselves.

This is the primary mechanism by which modern-day bacteria develop antibiotic resistance. If one primitive organism can develop a useful adaptation, other organisms can develop that same adaptation without having to evolve it from scratch.

The second major evolutionary step involves the development of specialized components within a single organism. The most primitive creatures have freely-floating bits of genetic material enclosed with some protoplasm inside a cell membrane, with nothing more specialized than that. These are the prokaryotic organisms of the world: the first forms of life thought to exist.

But more evolved creatures contain within them the ability to create miniature factories, capable of specialized functions. These mini-organs, known as organelles, herald the rise of the eukaryotes. Eukaryotes are larger than prokaryotes, have longer DNA sequences, but also have specialized components that perform their own unique functions, independent of the cell they inhabit.

These organelles include a cell nucleus, the lysosomes, chloroplasts, golgi bodies, endoplasmic reticulum, and the mitochondria. Mitochondria themselves are incredibly interesting, because they provide a window into life's evolutionary past.

If you take an individual mitochondria out of a cell, it can survive on its own. Mitochondria have their own DNA and can metabolize nutrients: they meet all of the definitions of life on their own. But they are also produced by practically all eukaryotic cells.

Contained within the more complicated, more highly-evolved cells are the genetic sequences that enables them to create components of themselves that appear identical to earlier, more primitive organisms. Contained within the DNA of complex creatures is the ability to create their own versions of simpler creatures.

In biology, structure and function is arguably the most basic relationship of all. If an organism develops the ability to perform a specific function, then it will have a genetic sequence that encode the information for forming a structure that performs it. If you gain that genetic code in your own DNA, then you, too, can create a structure that performs the specific function in question.

As creatures grew in complexity, they accumulated large numbers of genes that encoded for specific structures that performed a variety of functions. When you form those novel structures yourself, you gain the abilities to perform those functions that couldn't be performed without those structures. While simpler, single-celled organisms may reproduce faster, organisms capable of performing more functions are often more adaptable, and more resilient to change.

By the time the Huronian glaciation ended and Earth was once again a warm, wet world with continents and oceans, eukaryotic life was common. Prokaryotes still existed (and still do), but were no longer the most complex creatures on our world. For life's complexity to explode, however, there were two more steps that needed to not only occur, but to occur in tandem: multicellularity and sexual reproduction.

Multicellularity, according to the biological record left behind on planet Earth, is something that evolved numerous independent times. Early on, single-celled organisms gained the ability to make colonies, with many stitching themselves together to form microbial mats. This type of cellular cooperation enables a group of organisms, working together, to achieve a greater level of success than any of them could individually.

Multicellularity offers an even greater advantage: the ability to have "freeloader" cells, or cells that can reap the benefits of living in a colony without having to do any of the work. In the context of unicellular organisms, freeloader cells are inherently limited, as producing too many of them will destroy the colony.

But in the context of multicellularity, not only can the production of freeloader cells be turned on or off, but those cells can develop specialized structures and functions that assist the organism as a whole. The big advantage that multicellularity confers is the possibility of differentiation: having multiple types of cells working together for the optimal benefit of the entire biological system.

Rather than having individual cells within a colony competing for the genetic edge, multicellularity enables an organism to harm or destroy various parts of itself to benefit the whole. According to mathematical biologist Eric Libby:

cell living in a group can experience a fundamentally different environment than a cell living on its own. The environment can be so different that traits disastrous for a solitary organism, like increased rates of death, can become advantageous for cells in a group.

There are multiple lineages of eukaryotic organisms, with multicellularity evolving from many independent origins. Plasmodial slime molds, land plants, red algae, brown algae, animals, and many other classifications of living creatures have all evolved multicellularity at different times throughout Earth's history.

The very first multicellular organism, in fact, may have arisen as early as 2 billion years ago, with some evidence supporting the idea that an early aquatic fungus came about even earlier.

But it wasn't through multicellularity alone that modern animal life became possible. Eukaryotes require more time and resources to develop to maturity than prokaryotes do, and multicellular eukaryotes have an even greater timespan from generation to generation. Complexity faces an enormous barrier: the simpler organisms they're competing with can change and adapt more quickly.

Evolution, in many ways, is like an arms race. The different organisms that exist are continuously competing for limited resources: space, sunlight, nutrients and more. They also attempt to destroy their competitors through direct means, such as predation. A prokaryotic bacterium with a single critical mutation can have millions of generations of chances to take down a large, long-lived complex creature.

There's a critical mechanism that modern plants and animals have for competing with their rapidly-reproducing single-celled counterparts: sexual reproduction. If a competitor has millions of generations to figure out how to destroy a larger, slower organism for every generation the latter has, the more rapidly-adapting organism will win. But sexual reproduction allows for offspring to be significantly different from the parent in a way that asexual reproduction cannot match.

To survive, an organism must correctly encode all of the proteins responsible for its functioning. A single mutation in the wrong spot can send that awry, which emphasizes how important it is to copy every nucleotide in your DNA correctly.

But imperfections are inevitable, and even with the mechanisms organisms have developed for checking and error-correcting, somewhere between 1-in-10,000,000 and 1-in-10,000,000,000 of the copied base pairs will have an error.

For an asexually-reproducing organism, this is the only source of genetic variation from parent to child. But for sexually-reproducing organisms, 50% of each parent's DNA will compose the child, with some ~0.1% of the total DNA varying from specimen to specimen.

This randomization means that even a single-celled organism which is well-adapted to outcompeting a parent will be poorly-adapted when faced with the challenges of the child.

Sexual reproduction also means that organisms will have an opportunity to a changing environment in far fewer generations than their asexual counterparts. Mutations are only one mechanism for change from the prior generation to the next; the other is variability in which traits get passed down from parent to offspring.

If there is a wider variety among offspring, there is a greater chance of surviving when many members of a species will be selected against.

The survivors can reproduce, passing on the traits that are preferential at that moment in time. This is why plants and animals can live decades, centuries, or millennia, and can still survive the continuous onslaught of organisms that reproduce hundreds of thousands of generations per year.

It is no doubt an oversimplification to state that horizontal gene transfer, the development of eukaryotes, multicellularity, and sexual reproduction are all it takes to go from primitive life to complex, differentiated life dominating a world.

We know that this happened here on Earth, but we do not know what its likelihood was, or whether the billions of years it needed on Earth are typical or far more rapid than average.

What we do know is that life existed on Earth for nearly four billion years before the Cambrian explosion, which heralds the rise of complex animals. The story of early life on Earth is the story of most life on Earth, with only the last 550-600 million years showcasing the world as we're familiar with it. After a 13.2 billion year cosmic journey, we were finally ready to enter the era of complex, differentiated, and possibly intelligent life.

What Was It Like When Oxygen Appeared And Almost Murdered All Life On Earth?

Although it was more than 4½ billion years ago that planet Earth formed, it was just a few hundred million years later, at most, that life arose on our world. For all the years since then, it's thrived and evolved, enabling it to find a way to exist in practically every environmental niche that Earth possessed.

But 2 billion years after Earth first took shape, life almost ended. The atmosphere had slowly been altered by the gradual addition of oxygen, which proved to be fatal to the most common type of organism present on Earth at the time. For hundreds of millions of years, the Earth entered a horrific ice age which froze the entire surface: known today as a Snowball Earth scenario. It was a disaster that almost ended life on Earth entirely. Here's the story of our near-death and ultimate survival.

One of the simplest experiments you can do in biology class is to put a group of cells into a nutrient solution, like yeast in molasses. The organisms will initially become very successful, as food is abundant, there's no competition for resources, and they can easily survive and reproduce. If you count the living organisms inside, that number will start growing exponentially.

But, in short order, all of that will change.

Yeast consume food through the process of fermentation. The cells feed on sugar by converting it into alcohol, ATP (which gets used for energy), and carbon dioxide as a waste product. But if you have a liquid water solution and you add carbon dioxide to it, it forms carbonic acid. At some critical point, it becomes too acidic for yeast to survive, and the population crashes.

This might be a simple biological scenario, but its results are nearly universal. In the presence of virtually no competitors or predators, and given practically unlimited resources, a living population will grow at an exponential rate. It will consume the available resources, produce whatever metabolism products it produces, and then reproduce in greater-than-replacement-level numbers.

The next generation will then consume more, produce more of its metabolites, and reproduce in even greater numbers. So long as resources are freely available, this process will continue. Until, that is, the metabolic processes it has been undergoing build up to a critical level where it poisons its environment. If this sounds like what the yeast did — or what modern humans are doing with CO2 — you've put the pieces together correctly. Organisms, if left unchecked, will poison their habitat with the waste products of their own success.

But we are not the first to encounter this problem, nor were the much more primitive yeast cells. In the very early stages of our Solar System, a simple form of prokaryotic life arose: unicellular organisms. Although we don't know the properties of the hypothesized protocells that theoretically gave rise to the first unicellular organisms, there is clear evidence of unicellular bacteria by time the Earth was perhaps 500 million years old: around 4 billion years ago.

Evolution then went in many different directions, as expected, to fill every available ecological niche. Archaea arose, able to survive in the deep sea around hydrothermal vents. Plasmids, which carry genes responsible for novel abilities, arose as independent DNA molecules, unattached to the bacterial chromosome itself. And, hundreds of millions of years later, the first fully photosynthetic organisms came to be.

By the time we fast-forward to 3.4 billion years ago, the first evidence for photosynthesis in living organisms starts to appear. There are a number of different ways that photosynthesis can occur, but all involve sunlight of a particular wavelength striking a molecule that can absorb it, exciting an electron that can then have its energy used in life processes.

Many organisms, such as green and purple sulfur and nonsulfur bacteria, make use of a variety of molecules to provide the electrons in their reactions, such as hydrogen, sulfur, and numerous acids. But organisms also evolved that use water as electron donors: the cyanobacteria, known as blue-green algae. Unlike the other (generally, but not universally, thought to be earlier) organisms, cyanobacteria produce molecular oxygen as a waste product.

Cyanobacteria still survive today, and are the only photosynthetic prokaryotes that produce oxygen. They seem to be more evolved than the other, non-oxygen producing photosynthetic prokaryotes. These blue-green algae possesses internal membranes (unlike the others), and are known to have arisen no later than 2.5 billion years ago.

The evidence we have is straightforward: right around that time, Earth's atmosphere began to display evidence for having free oxygen present within it. Slowly but surely, the oxygen content began to build, and an organism with a seemingly unlimited resource — sunlight — began to poison its environment. Oxygen, you see, is not just corrosive and flammable; it's also the cause of the greatest climate disaster in history: the Huronian Glaciation.

The cyanobacteria, experiencing massive successes, evolved into microbial mats in short order, and that early presence of atmospheric oxygen systematically removed the early methane from Earth's atmosphere.

The conversion of methane into carbon dioxide and water greatly decreased the greenhouse effect from Earth's early atmosphere.Concurrently, the oxygen produced by the cyanobacteria killed off most of the other, non-oxygen-using life forms, as oxygen was toxic to them.

Considering that the Sun's energy output was much lower in the early stages, this large amount of methane was the only thing keeping Earth as a relatively temperate planet. With the oxygen destroying that powerful greenhouse gas, the planet couldn't retain its heat as well. The greatest ice age in history, which led to Snowball Earth conditions for approximately 300 million years, was now upon us.

Although the exact ratios of the different atmospheric components of Earth are unknown, there were... [+] large amounts of methane present in the atmosphere prior to 2.5 billion years ago and virtually no oxygen. With the arrival of oxygen, the methane was destroyed, and the planet's greatest ice age began.

As catastrophic as this was for life on Earth, though, it didn't end it. The cyanobacteria continued to thrive, while smaller populations of other organisms, facing tremendous selection pressures and a changing environment, evolved in a different direction. More complex creatures, accumulating large numbers of genes and novel abilities, stood better chances of surviving, as they were more resilient to change.

Meanwhile, volcanoes continued to erupt beneath the ice. These build-ups of carbon dioxide could increase the greenhouse effect in the atmosphere, while the simultaneous production of ash could decrease the reflectivity of Earth, allowing us to eventually break out of this glaciation era.

While it lasted for approximately 300 million years, the end of the Huronian Glaciation coincides with the first evidence we have for eukaryotic life. Cells now existed that had enclosed, separated organelles that could carry out independent functions. Eukaryotes would later give rise to all the extant protists, plants, fungi and animals that exist today; it's arguable that human-like life would never have arisen if oxygen had never destroyed our methane-rich atmosphere and led to this ancient, Snowball Earth scenario.

This period of time in Earth's history may have been the greatest mass extinction our planet has ever faced. Yet even at this primitive stage, life remained ubiquitous and resilient, and the destruction of the existing, dominant species allowed other, new organisms to evolve and rise to fill the vacant ecological niches.

The Great Oxygenation Event was a transformative occurrence in Earth's history. Without it, life may never have become complex, differentiated, and capable of giving rise to intelligent organisms like us.

What Was It Like When Planet Earth Took Shape?

A little over 4.5 billion years ago, our Solar System began to form. Somewhere in the Milky Way, a large cloud of gas collapsed, giving rise to thousands of new stars and star systems, each one unique from all the others. Some stars were much more massive than our Sun; most were much smaller. Some came with multiple stars in their systems; about half the stars formed all by their lonesome, like ours did..

But around practically all of them, a large amount of matter coalesced into a disk. Known as protoplanetary disks, these would be the starting points for all the planets that formed around these stars. .

With the advances in telescope technology that's accompanied the past few decades, we've started to image these disks and their details firsthand. For the first time, we're learning how planetary systems like our own came into existence.

In theory, the process of forming planets is incredibly straightforward. Whenever you have a large mass, like a gas cloud, you can expect the following steps to happen: the mass gets drawn into a central region,
where one or more large clumps will grow,
while the surrounding gas collapses,
with one dimension collapsing first (creating a disk),
and then imperfections in the disk grow,
preferentially attracting matter and forming the seeds of planets.
We can now look directly at these protoplanetary disks, and find evidence that these planetary seeds are present from a very early time.

But these disks won't last very long. We're looking at timescales that are typically only tens of millions of years long to form planets, and that's due to not only gravitation, but to the fact that we've got at least one central star shining as well. The cloud of gas that will form our planets is made out of a mix of elements: hydrogen, helium, and all the heavier ones, going way up the periodic table. When you're close to the star, the lightest elements are easy to blow off and evaporate. In short order, a young solar system will develop three different regions: a central region, where only metals and minerals can condense into planets,
an intermediate region, where rocky and giant worlds with carbon compounds can form,
and an outer region, where volatile molecules such as water, ammonia, and methane can persist.

The border between the inner two regions is known as the Soot Line, where being interior to it will destroy the complex carbon compounds known as polycyclic aromatic hydrocarbons.

Similarly, the border between the outer two regions is known as the Frost Line, where being interior to it will prevent you from forming stable, solid ices. Both lines are driven by the heat of the star, and will migrate outward over time.

Meanwhile, these protoplanetary clumps will grow, accrete additional matter, and will have opportunities to gravitationally perturb one another. Over time, they can merge together, gravitationally interact, eject each other, or even hurl one another into the Sun. When we run simulations that allow planets to grow and evolve, we discover an extraordinarily chaotic history that's unique for each and every solar system.

When it comes to our own Solar System, the cosmic story that unfolded was not only spectacular, it was in many ways unexpected. In the internal region, it's very likely that we had a relatively large world present early on, which was possibly swallowed by our Sun in our cosmic youth. There is nothing preventing a giant world from forming in the inner Solar System; the fact that we have only the rocky worlds close to our Sun tells us that something else was likely present early on.

The largest planets probably formed from seeds early on, and there may have been more than four of them. In order to get the present configuration of gas giants, the simulations we run seem to show that there was at least a fifth giant planet that was ejected at some point long ago.

The asteroid belt, between Mars and Jupiter, is very likely the remnants of our initial Frost Line. The border between where you can have stable ices should have led to a large number of bodies that were a mix of ice and rock, where the ices mostly sublimated away over the billions of years that have passed.

Meanwhile, out beyond our last gas giant, the leftover planetesimals from the Solar System's earliest stages persist. Although they may merge together, collide, interact, and occasionally get hurled into the inner Solar System from gravitational slingshots, they largely remain out beyond Neptune, as a relic from the youngest stages of our Solar System. In many ways, these are the pristine remnants from the birth of our cosmic backyard.

The planetesimals from the portions of the Solar System beyond the Frost Line came to Earth and made... [+] up the majority of what is our planet's mantle today. Out beyond Neptune, these planetesimals still persist as the Kuiper belt objects (and beyond) today, relatively unchanged by the 4.5 billion years that have passed since then.

But the most interesting place of all, for our purposes, is the inner Solar System. There may have once been a large, interior planet that was swallowed, or perhaps the gas giants once occupied the inner regions and migrated outwards. Either way, something delayed the formation of planets in the inner Solar System, allowing for the four worlds that did form — Mercury, Venus, Earth, and Mars — to be much smaller than all the others.

From whatever elements were left, and we know they were mostly heavy ones from the planetary density measurements we have today, these rocky worlds formed. Each one has a core made of heavy metals, accompanied by a less-dense mantle made out of material that fell onto the core later, from beyond the Frost Line. After only a few million years of this type of evolution and formation, the planets were similar in size and orbit to how they

But there was a huge difference: in these early stages, Earth didn't have our Moon. In fact, Mars didn't have any of its moons, either. In order for this to occur, something needed to create them. That would require a giant impact of some type, where a large mass struck one of these early worlds, kicking up debris that eventually coalesced into one or more moons.

For Earth, this was an idea that wasn't taken particularly seriously until we went to the Moon and investigated the rocks we found on the lunar surface. Quite surprisingly, the Moon has the same stable isotope ratios that the Earth does, while they're different between all the other planets of the Solar System. Additionally, the Earth's spin and the Moon's orbit around Earth have similar orientations, and the Moon has an iron core, all facts which point to a mutual common origin for the Earth and the Moon.

Originally, the theory was called the Giant Impact Hypothesis, and was theorized to have involved an early collision between proto-Earth and a Mars-sized world, called Theia. The Plutonian system, with its five moons, and the Martian system, with its two moons (that likely used to be three), all show similar evidence of having been created by giant impac

But now, scientists are noticing problems with the Giant Impact Hypothesis as originally formulated for creating Earth's Moon. Instead, it looks like a smaller (but still very large) impact, from an object originating much farther out in our Solar System, may have been responsible for the creation of our Moon

. Instead of what we call a giant impact, a high-energy collision with proto-Earth could have formed a debris disk around our world, creating a new type of structure known as a synestia.

. There are four big properties of our Moon that any successful theory for its origin must explain: why there is only one large moon rather than multiple moons, why the isotope ratios for elements are so similar between the Earth and Moon, why the moderately volatile elements are depleted in the Moon, and why the Moon is inclined as it is with respect to the Earth-Sun plane.

. The isotope ratios are particularly interesting for the Giant Impact Hypothesis. The similar isotopic properties between the Earth and Moon suggest that the impactor (Theia) and Earth, if they were both large, had to be formed at the same radius from the Sun.

. This is possible, but models that form a Moon via that mechanism don't give the right angular momentum properties. Similarly, grazing collisions with the right angular momentum give rise to different isotopic abundances than what we see.

That's why the alternative — a synestia — is so appealing. If you have a fast, energetic collision between a smaller body that's less massive and our proto-Earth, you'd form a large torus-shaped structure around the Earth. This structure, called a synestia, is made of vaporized material that originated from a mix of proto-Earth and the impacting object.

Over time, these materials will mix, forming many mini-moons (called moonlets) in short order, which can stick together and gravitate, leading to the Moon we observe today.

Meanwhile, the majority of the material in the synestia, particularly the inner part, will fall back to Earth. Rather than a single, contrived giant impact, we can now speak in terms of generalized structures and scenarios that give rise to large moons like our own.

There was almost certainly a high-energy collision with a foreign, out-of-orbit object that struck our young Earth in the early stages of the Solar System, and that collision was required to give rise to our Moon.

But it was very likely much smaller than Mars-sized, and it was almost certainly a sturdy strike, rather than a glancing collision. Instead of a cloud of rock fragments, the structure that formed was a new type of extended, vaporized disk known as a synestia. And over time, it settled down to form our Earth and Moon as we know them today.

At the end of the early stages of our Solar System, it was as promising as it could be for life. With a central star, three atmosphere-rich rocky worlds, the raw ingredients for life, and with gas giants only existing much further beyond, all the pieces were in place. We know we got lucky for humans to arise. But with this new understanding, we also think the possibility for life like us has happened millions of times before all throughout the Milky Way.

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