Who are we?

06

By John Wenz

Evolution is often a true case of “survival of the fittest”—and out of all of our relatives, we were the one able to most withstand the trials and tribulations set before us.

But there’s no easy roadmap for how we got here, and the echoes of other earlier species live within our DNA. Whereas the prevailing view used to be that humans arose out of Africa and moved out into the greater world as one monolithic species, the view over the last few years has been something radically different: that several species spread out over the world, interacting and interbreeding with each other, giving a messy rise to what we call Homo sapiens today.

Indeed, some people of European and Asian descent have around 2 percent Neanderthal DNA hiding in their genes. But a 2010 announcement of a new, previously unknown, closely related species called the Denisovans came with another announcement: some people in the Philippines, Australian aborigines, Melanesians, and Papuan people share 5 to 6 percent DNA with the Denisovans.

There’s an even stranger part to that story … we have no idea what Denisovans look like! The fossil evidence so far for them are two teeth, a small finger bone, and an unidentifiable splinter off of a large bone. We only know that to be the case because of the DNA evidence, more richly preserved than any Denisovan fossils.

So what, exactly, is the history of humanity? We’re currently working on having an answer for that, but it’s a story with twists, turns, and incomplete chapters. The history of human evolution is changing rapidly through the work of paleogeneticists, who look for DNA in ancient fossils and try to place animals on the tree of life in ways unimaginable 100 years ago.

The Beginning

Humans are, for all intents and purposes, a very smart cousin of chimpanzees. They belong to a group of primates called the great apes. While many species have come and gone, there are five species of Great Ape today: gorillas, orangutans, chimpanzees, bonobos, and humans. The scientific term for the great apes family is Hominidae.

打印Related species don’t just arise out of the blue one day. Instead, closely related modern species share a common ancestor, a species that branched out into other species that eventually led to the apes we see today. Orangutans are the most distantly related to the other great apes, having split off evolutionarily about 13-17 million years ago. Gorillas and chimps split off from a common ancestor 8 million years ago, then humans and chimps split off about 5 million years ago.

Hominidae in turn belong to Hominoidea, which is a big umbrella term for apes. This group includes gibbons, a grouping of 16 species of small apes distributed throughout southeast Asia. Some are monkey-sized, but lack tails. We split from Gibbons 17-18 million years ago. Hominoidea, in turn, split off from Old World monkeys 25 million years ago. Old World monkeys include baboons, macaques, and mandrills, consisting of Asian and African monkeys that lack prehensile tails. There are lots of other primates more distantly related to us, All primates, in turn, come from a common ancestor with rodents, which split off 80-100 million years ago when dinosaurs still roamed the Earth. The first primates actually ascended at the time of the dinosaurs, about 66 to 85 million years ago. This means that the far-ancestors to the apes and monkeys were probably eaten by Velociraptors, which lived in the same era.

If you trace it even further back, Euarchonta (the earliest primates / tree shrews / flying lemurs) are all part of the descendants of Synapsids, the reptilian ancestors of all mammals. If you’ve been to a museum and seen a dimetrodon, that fearsome lizard with a giant sail on its back, you’re looking at an incredibly distantly related cousin. Synapsids first broke off from the ancestor of birds and modern reptiles roughly 256 million years ago: before the Triassic period, when dinosaurs rose up.

打印Fossils

Discovering the common pre-divergence species is a hard undertaking. The fossil record of primates is difficult to figure out because the fossil record is incomplete. Not every bone fossilises. In fact, according to the BBC, less than one percent of species that have ever lived became fossils. A carcass doesn’t survive the elements well. Scavengers may pick at the bones in search of meat. Water or weather can erode away the bones. You need to be covered, quickly, in mud or otherwise encased in some way so that scavengers and weather don’t erode your bones into nothingness.

Primates tend to live in forests and other wet environments where fossilisation is a rare feat. This means that the primate evolutionary tree is filled with holes that may never be filled. It’s entirely possible that some species left behind no bones at all, while others may leave behind just a few unidentifiable trace bones like teeth, a skull fragment, or a small bone.

This also means that the stepping stones to finding out what it means to be human are incomplete. We have bits and pieces to assemble something of a view, but some species in the middle are virtually gone from the fossil record.

The closest species in the fossil record showing a split between the great apes is the Pierolapithecus catalaunicus, an arboreal ape that lived in Spain 13 million years ago. It shows more in common with gorillas, chimps, and humans than it does the orangutan. There is some debate as to if it’s an exact match as an ancestor or just closely related—especially given its European origins, whereas the gorillas, chimps, and humans all came up in Africa—but nonetheless it seems closely related to such an ancestor. Three million years later, the ancestor of humans and chimps split from the ancestor of the gorilla. The lineage that led to humans and the lineage that led to chimps split off shortly after.

Then … there are bits and pieces in the fossil record. Like early humans, early proto-human species may have been closely related enough to interbreed with moderately distant ancestors. Some scientists believe the genus Sahelanthropus may be more closely related to humans than chimps, while others think it could be the common ancestor between the two. It is known only from a few skulls.

One genus, Ardipithecus, may be a direct ancestor to humans, but even it has been under debate. It still shares many chimp-like characteristics. The idea is that human ancestors, unlike chimp ancestors, were able to walk bipedally in order to move beyond trees and into more open terrain. The Ardipthecus specimens have ambiguous evidence for bipedalism, as does Sahelanthropus. Bipedalism means to walk on two feet at all times, which only adult humans do. Orangutans are the only great ape to spend most of its time in trees and thus can still brachiate, or swing between branches. Gorillas and chimps are “knuckle walkers,” who move on all fours, using the outside of their hand (rather than the inner palm, as on other animal paws) to walk or run.

The species most widely believed to be an ancestor to humans is the Australopithecus afarensis. One specimen of the species that’s become especially famous is Lucy, a 3.2 million year old Australopithecus skeleton named for The Beatles’ song “Lucy in the Sky with Diamonds.” Lucy’s skeleton is only partially preserved—owing to how hard it is for a bone to survive the elements—but is intact enough to determine that she was fully bipedal. Scientists have actually taken what bones remain to create reconstructions of Lucy. It is believed that Australopithecus arose 3.9 million years ago and lasted for around 1 million years as a species.

There is an asterisk to her story, though. A fossil of the genus Orrorin shows a more advanced hip structure than Lucy, whose hip bones were more akin to chimps. That could make Orrorin, whose skeleton is less complete, a more direct ancestor to humans and make Lucy more of a distant cousin, but not enough is known about Orrorin to decide for sure.

A relative to Australopithecus, Kenyanthropus platyops, lived around 3.5 million years ago and shows evidence of tool use. There is scant evidence that Lucy and her species-mates used tools, so this, if true, would be the earliest evidence of tool use in a human ancestor.

But 2.5 million years ago, everything changed. The first true primitive humans arrived on the scene.

Early humanity

The first fossil of Homo habilis was dug up in Tanzania in 1955. Like the Denisovan, it was discovered through just two teeth by Mary Leakey, an expert in human evolution and archaeology. Leakey didn’t know what species the teeth belonged to until 1959, when she found a skull with molars so big it was nicknamed “the Nutcracker Man.” It shared a lot of features in common with Australopithecus, but had a more human-like face and used tools. In fact, “Homo habilis” literally translates into Handy Man, a name given to it due to its extensive tool production use.

It was believed that Habilis gave rise to Homo ergaster, and Ergaster was in turn the ancestor of Homo erectus. Homo erectus means “upright man,” though it’s likely not the earliest human relative with an upright posture. A 1.8 million year old foot of Homo habilis called OH 8 seems to show that it walked fairly similar to us.  But there are some twists and turns here too. Habilis, ergaster, and erectus all lived on Earth at the same time. This could mean that they were all descended from a yet-undiscovered, more ancient ancestor. Habilis also showed signs of being more like Australopithecus than later Homo skeletons, so it may not be so “human” after all. But there are others who think that habilis, ergaster, and erectus are actually natural variations in specimens of the same species living in different regions. Homo rudolfensis is also part of this hypothesis. Habilis, in this contrasting view, may be just one of the earliest examples of Homo erectus.

One problem is that there are vast anatomical differences between members of the same species. Think of all the differences between house cats—some are big, some are small, some are very fluffy, some are bald. In the same way, some humans are tall, some are short. The tallest man alive right now is Sultan Kösen, who is 2.49 meters tall. The smallest adult human on Earth is just under a half meter. Different groups of people inherit traits—larger noses or smaller chins or more compact height—from their ancestors. Thus, if you only based your analysis of members of the same species based on three wildly different skeletons, they might actually be mistaken for different, but related, species.

This, in fact, might have happened before! There’s some debate as to whether three species of dinosaur—the famous triceratops and the less famous Torosaurus and Nedoceratops—are actually the same species at different ages. Without being able to peer back in time or gather more fossil evidence, that may be hard to figure out.

Homo habilis arose around 2.5 million years ago. A 2.1 million year old fossil site found in China indicates that it didn’t take long for early humans to spread throughout the world. This could have been the work of the Homo erectus, especially if it and Habilis were one and the same. Erectus has been found throughout Europe, Africa, and all throughout South Asia, stretching from the Middle East to China. This widespread distribution also gave its population time to grow to healthy numbers. The earliest confirmed erectus fossil comes from around 1.8 million years ago, while the latest fossil confirmed to be erectus date to about 150,000-250,000 years ago. That means that, for a small period of time, Homo sapien and Homo erectus overlapped!

The Denisovan DNA story is even more complicated when you get into the particulars: it showed not just Denisovan ancestry, but also a little bit of Neanderthal and something even more ancient and primitive. The DNA may belong to Homo erectus, though no usable sample has ever been obtained to confirm this.

1.2 million years ago, Homo antecessor appeared. Both fossils that have been found of this species were found in Northern Spain. Antecessor is believed by many to be the common link between humans and Neanderthals, a view upheld by genetic evidence surrounding Homo heidelbergensis. Heidelbergensis was once believed to be a common ancestor between Neanderthals and humans, but a femur bone of the species found in a cave in the Iberian Peninsula showed that it was more closely related to Neanderthals and Denisovans.

But there are some people, too, who suggest that Antecessor could be another regional variation on the widespread Homo erectus. There’s also another strange part to this story: antecessor has only been found in Spain, but Homo sapien, by all available evidence, first arose as a species in Africa. Many early Homo sapien fossils have been found in Ethiopia, though one contentious fossil—from the Jebel Irhoud site in Morocco—may be the earliest Homo sapien skull ever discovered, at 315,000 years old. If sea levels at the Iberian Peninsula—which nearly connects Spain to Africa—were at the time low enough to create a landbridge between the two, it may have allowed one group of Antecessor to migrate into Africa and become Homo sapiens while another group stayed behind and gave rise to Neanderthals and Denisovans.

The Big Jumble

Once Homo sapiens arrived on the scene, things got interesting: at least six species of humans had spread across the world. Homo erectus lasted until 250,000 years ago, though some fossils show even later survival, up to 200,000 years ago. Neanderthals inhabited much of Europe, while their cousins, the Denisovans, seemed to have spread into Asia. Homo sapiens were creeping up toward the Middle East. Then there are two quirky human cousins that we’re still figuring out: Homo naledi and Homo floresiensis.

Homo naledi is a relatively recent addition to the human family tree. The first fossils were found in a cave in South Africa in 2013. The cave is hard to access, so the team was composed of several small statured anthropologists able to navigate the Rising Star Cave system. Two excavations, one in 2013 and one in 2014, managed to bring out 1,550 bones and other artifacts from the cave. Once researchers began to examine the fossils, they realised they were looking at a never-before-identified human species.

The species has a lot of archaic traits, such that some parts of their anatomy almost resemble the Australopithecus, especially in the hip area, while in other ways, they seemed closer to a Homo erectus relative. Initial dating that places the bones at 2 million years old made sense in this context, but an updated dating of the bones placed them at closer to 250,000 years ago. Scientists first made the determination of Naledi’s age based on the similarity of its facial features to Australopithecus and H. erectus. But later, better fossil samples were able to give a radiocarbon dating of closer to 250,000 years. This means that Homo sapiens and naledi overlapped, and that some of the most ancient lineages of humans survived long after they’d otherwise been supplanted.

The other species, Homo floresiensis, lived in the Island of Flores in Indonesia lived from about 190,000 years ago to about 50,000 years ago, though the fossils were once believed to be much younger (12,000 years ago.) They’re notable for their short stature—standing at just 1 meter tall—and their relatively late survival. Like Naledi, some features are as old as the Australopithecus lineage. This includes the wrist bones and the hip bones. Their small stature is caused by their environment—oftentimes, populations of animals isolated on islands can take on a comparatively small stature to their continental-dwelling relatives if nutritional resources are scarce.

While there’s been a lot of debate as to the Flores Man’s origin (they are sometimes called “Hobbits” after the short creatures in J.R.R. Tolkien’s Lord of the Rings books), the emerging picture is that they may be one of the oldest human lineages, possibly coming before Homo erectus. Because they were found in Southeast Asia, this means that even the most primitive human species were wanderers, moving beyond their African origin to a worldwide dispersal, powered by their intelligence, adaptability, and ability to walk on two feet.

The problem, as pointed out by a Nature Education article in 2012, is that the Flores Man seems to have features even a bit primitive by Homo erectus standards. Its skull closely resembles Homo erectus or Homo habilis, but its skeleton is more of an evolutionary throwback to Lucy and her fellow Australopithecus. The wrist, for instance, is far more primitive than any other member of the Homo genus, and the feet would mean the Flores Man walked differently than other members of Homo—whereas even early members like Homo habilis walked with the same gait as modern humans. This could place it right as a descendant of some relative right at the boundary between Lucy and Homo habilis.

Homo Sapiens

It was once, in the absence of better fossil evidence, assumed that humans left Africa all at once. But several fossils seem to refute that claim, showing widespread distribution across Europe, Asia, and Africa. That doesn’t just go for Homo erectus either, but also the Denisovans and the Neanderthals. Homo sapiens, as the seeming latecomers, seem to have started out in Africa—however our species came up—and moved into the Levant by about 175,000 years ago, according to the oldest fossils of Homo sapiens found in Israel. Their spread was slow, and there may have been multiple migration events. The most widespread dispersal—the one that truly took us to other continents to stay—began about 70,000 years ago.

50,000 years ago, Homo sapiens and Neanderthals first interbred in modern day Israel. Sapiens interbred with the Denisovans sometime after that. The cohabitation wasn’t long lived—indeed, at certain points, Neanderthals and Homo sapiens had drifted so far apart that a Homo sapien woman couldn’t carry a Neanderthal child to term, but a Neanderthal mother and a Homo sapien father could procreate.

About 50,000 years ago, the Flores Man is believed to have died out, barring some kind of new fossil evidence that conclusively places them at 12,000 years ago.

44,000 years ago, Homo sapiens interbred with Denisovans in Southeast Asia, their DNA permanently imprinting there. There may even have been two separate events. The elusive Denisovans may not have left a fossil record, but it did leave behind a DNA scavenger hunt.

There’s also the question of more “Archaic” humans, including possibly Homo erectus. We don’t have DNA samples directly from an erectus fossil, but Denisovans show evidence of an admixture with a group only distantly related to humans and Neanderthals. This is perhaps the biggest mystery in our past, as a 2011 paper showed that an ancient human species may have interbred with sub-Saharan Africans 35,000 years ago—long after we’d assumed they died out, showing that Homo erectus or something like it may have been amazingly resilient. Whatever the lineage was, it had split off from Homo sapiens 700,000 years ago.

And that’s the thing—there may have been numerous species that came and went, their fossils gone forever due to conditions too dry or too humid or too inhospitable to preserve the story of what happened to them. The story of human evolution has been put together in bone fragments, incomplete skeletons, elusive DNA, and leftover tools. We became human through fits and starts and migrations throughout the world, interactions with cousin species.

Some of those species aren’t even well known enough to place on our family tree. A fossil called Penghu 1 found in Taiwan may represent some of the earliest modern humans in Asia, or the jaw fragment may just be a lost cousin. There’s an Ethiopian fossil, the Herto Man, who has been assigned their own separate subspecies, Homo sapiens idaltu, because they still bore a few traces of ancient ancestors. A whole group of early human “species” have been folded into the robust Homo erectus lineage. A group of people who lived in China 11,000 years ago called the Red Deer Cave People seem to be modern humans, but with anatomical variations only seen in more ancient cousins. Callao Man, who lived in the Philippines 67,000 years ago, arriving by boat, can’t be placed as it’s little more than a finger bone. Homo gautengensis, an ancient cousin, doesn’t yet have a place on a tree, but could be older than H. habilis. And there are countless fossils too incomplete to assign a species. Had the Max Planck Institute not tested the DNA in the tooth and finger bone found in the Denisova cave, it might have been shelved away as another Neanderthal or Homo sapien fossil.

How did we become human? From fits and starts and interactions with our cousins, some of them resulting in hybrid babies. It’s a complicated, messy picture, one that’s coming into view now—but just getting more confusing as it does. It also shows that the human family “tree” is more of an overgrown bush of twisting and intertwining branches, hard to separate out from one another. Perhaps what we today call the main Homo sapien genome was a genome built out of closely related species contributing to our DNA as we expanded out into the world before we became the last surviving Homo. The picture is far from complete—but the mysteries have only deepened the more we’ve learned.