The Humans That Lived Before Us

For about a million years, the early Pleistocene (about 2.4 million to 1.4 million years ago), it was a really exciting time to be a hominid. Hominids, as you may recall, are the group of human ancestors that are more closely related to us than to chimpanzees and bonobos. During this million-year span, different branches of our evolutionary tree flourished throughout southern and eastern Africa. And if we were to approach the first part of this million years, we would find a familiar face. Or at least a somewhat familiar face: the face of Homo habilis. It was just over a meter tall and had a slightly larger brain and smaller teeth than its earlier relatives, the australopithecines.

But it still had longer arms and a protruding lower face, traits generally considered more basic in the hominid lineage. And yet! This ancestor probably made and used stone tools! In fact, Homo habilis means “skillful man,” and his discoverers gave him that name because they thought he was responsible for the numerous tools that had been found near his remains. But does this hominid really belong to our genus, the genus Homo? Was it more like us than its previous ancestors? Over the last fifty years or so, the human family tree has really filled out. We've discovered all kinds of new fossils of our ancestors and relatives, such as Australopithecines, which have about the same brain size and limb proportions as Homo habilis.

And this has led some researchers to question whether Homo habilis is actually a member of our genus. As more and more fossil ancestors have been found, our genus has become increasingly inclusive, incorporating more members who look less like us, Homo sapiens. And this is an important problem to think about. Because there is some consensus about who belongs in our immediate human family, like us, the Neanderthals, and even the ancient hominid Homo erectus, who traveled the world. They are all accepted to be clearly defined members of the genus Homo. But beyond them, there are many ancestors for whom we cannot find a home.

And there is also no official definition of what constitutes a human being, whether that means being a member of our genus or our species, or simply being able to walk upright and make tools. So by getting to know these other hominids (the ones before us, the Neanderthals, and our other contemporaries) we can begin to answer some important, interesting, and difficult questions... ...questions about what it essentially means to be. human. When the team of Louis and Mary Leakey found the first Homo habilis fossils in the 1960s, they had to make a difficult decision: were they the remains of australopithecines?

Or were they actually the first known members of our own genus, Homo? Traditionally, defining who belongs to our gender has come down to what traits are considered “uniquely” human. And when the Leakeys were reflecting on Homo habilis, they used a definition of Homo from 1955, which said that to be a member of the genus, you had to have a number of characteristics in common with the three members of Homo known at the time: Homo sapiens, Homo erectus and Neanderthals. The Leakeys decided that Homo habilis shared three important traits with other members of our genus: it had an upright posture;

He was bipedal and had manual dexterity to make tools. And, of course, Homo habilis had those three things. But in the decade after the discovery of Homo habilis, new discoveries were made of other human ancestors in the same parts of Africa, who also had these traits. And these new finds were all of various australopithecines, which were indisputably not part of our genus. The most famous of these discoveries is the specimen known as Lucy. Unearthed in Hadar, Ethiopia, in 1974, it was one of the most complete specimens of Australopithecus afarensis ever found. And it gave clear evidence of an upright posture, such as having the thigh bones sloping inward toward the knee and a more human-like pelvis.

Then, four years later, a set of fossilized footprints was found in Tanzania. Known as the Laetoli footprints, they were probably also made by Australopithecus afarensis, proving once again that hominids walked on two feet more than a million years before Homo habilis existed. So if walking upright wasn't unique to our gender, then the definition of our gender had to change. Rather than being limited to physical traits, thinking focused on lifestyle adaptations as a way to define who belonged in our group. Lifestyle adaptations are characteristics that are linked to how a hominid

lived

his life, such as what he ate, how he moved and where he

lived

.

For example, increased brain size in members of Homo was thought to be related to a higher quality diet, because being able to consume more calories more efficiently has led to larger brains. And some researchers came up with four specific lifestyle adaptations that they thought might qualify a hominid to enter the genus Homo. Those adaptations included: an adult brain size greater than 600 cubic centimeters; limb proportions similar to ours, with long legs compared to our arms; the use of language; and the manufacture and use of stone tools. But still, these things only apply to Homo habilis. Because one of the most famous and complete Homo habilis skulls, a specimen known as KNM-ER 1813, had a cranial capacity of only 510 cc.

Meanwhile, a large male specimen of Australopithecus afarensis was found to have similar limb proportions to early members of Homo, but lived 3.58 million years ago, long before Homo habilis appeared on the scene. And the capacity for language can only really be inferred from the fossil record. It is quite difficult to know whether Homo habilis or some ancestor who lived millions of years ago could speak. That only leaves the stone tools. And although researchers in the 1960s were pretty convinced that Homo habilis was the tool maker at Olduvai Gorge, we now know that Australopithecines could probably make stone tools as well.

So let's look at our group differently. Instead of talking about who might not belong to our gender, let's consider who might belong to our gender. Who were those other members of our genus who lived alongside Homo habilis during that exciting period of millions of years in Africa? And what can they tell us about the origin of the genus Homo? Well, approximately 1.98 million years ago, in South Africa, there lived an Australopithecus with features clearly similar to Homo. Known from several relatively complete skeletons, it was named Australopithecus sediba in 2010. Its discoverers placed it in Australopithecus because of its small brain and long arms, but they also noted that it had small molars and premolars, and facial features similar to other Homo specimens.

So these researchers actually think that Australopithecus sediba might be more closely related to our genus than other australopithecines, but other experts think it's too recent. Another candidate for inclusion? Homo rudolfensis has been found at sites dating between 1.8 and 1.9 million years ago in East Africa. The best fossil of this species is known as KNM-ER-1470, and when it was discovered in 1972, it was originally classified as a large specimen of Homo habilis. However, in 1986 and again in 1992, additional studies found that its larger brain, longer face, and larger premolars and canines made it too different from Homo habilis to be a member of that species.

But he was still assigned to our genus because of his big brain. At 775 cubic centimeters, it was well above the classic limit of 600 cc. And finally we come to the first indisputable member of our genus, and one of the most successful and widespread: Homo erectus. He lived from 1.9 million years ago to just 143,000 years ago! The first fossils of Homo erectus were found in 1891, and some anthropologists later divided this species into two: Homo erectus included the later African and Asian fossils, and the earlier African fossils were classified as Homo ergaster. And experts generally agree that Homo erectus is definitely a member of our genus.

These hominids had similar proportions to modern

humans

, were potentially capable of long-distance running, and generally had much smaller molars and much larger brains than their predecessors. In other words, they were much more like us than any of the other species I've mentioned so far. Homo erectus is also the first species for which we have fossil evidence outside of Africa. They reached as far as China and Indonesia, but their initial foray appears to have taken them to the Republic of Georgia, at a site called Dmanisi that dates back approximately 1.77 million years. And the interesting thing about that site is that there is a lot of variation among the specimens found there.

Some Dmanisi individuals had the unmistakable brow ridge of Homo erectus, but their brains measured less than 600 cc, the classic limit for their inclusion in the genus Homo. In fact, there is so much variation in Georgian fossils that their discoverers advocated in 2013 for taking all other early Homo fossils, including those assigned to Homo habilis and Homo rudolfensis, and placing them in Homo erectus, grouping everything together as a single species. They argue that if fossils from a single site can show as much variation as we find between species, then all of those early groups could also be considered the same species.

But of course other experts disagree. They do not believe that the overall shape of the skull is sufficient to distinguish one species from another. For them, the devil is in the different morphological details of each skull. Now, with all this in mind, let's return to Homo habilis. Where it belongs? Well, it doesn't really seem to fit anyone's definition of our gender. And the best argument for keeping it is that removing it would require redefining what it means to be a member, which would be a major taxonomic task. Some experts have proposed grouping habilis in the genus Australopithecus.

Others say it is neither Homo nor Australopithecus and deserves its own new genus. So far no single opinion has prevailed. Homo habilis remains a taxon in limbo. Ultimately, what defines our gender comes down to how much variation in morphology, time, and space we are willing to include in the group we call home. In the past, an increase in brain size, bipedal walking, human limb proportions, and tool use seemed to have been sufficient for inclusion. Those are the things we thought made us members of the same gender. But as we've discovered more and more hominid fossils, our family tree has become more, rather than less, complicated.

Now, the latest research suggests entirely new ways to define our lineage. A new idea for a defining characteristic of our genre? Tooth size! Smaller teeth generally indicate a higher quality diet and the ability to prepare food with tools, rather than having to chew hard foods for a long time. Another possible criterion is the pace of our development. Modern

humans

have longer periods of childhood and adolescence compared to our closest ape relatives, because we need that time to grow our big brains and use them to learn. And we can trace these growth patterns in fossils by studying the microscopic features of teeth.

And as recently as 2015, some experts suggested that we should eliminate the entire hominid list and start from scratch. They say we should take a step back and look at the entire fossil record with new eyes to decide what traits we think are important for being "human." As things stand, there is still no single way to define our gender. It happens mainly by comparison: is a new fossil more like what we called Homo in the past or more like an australopithecus? And the jury is still out on Homo habilis, the species that started all the problems. But in any case, the problem really began back in the early Pleistocene, during that exciting million years in which this group of hominids began to flourish.

And it may be that fossils from that era (perhaps in fossils we haven't found yet) will help us better answer the question of who belongs to our very exclusive group. Thanks as always and many thanks to our current eontologists, Jake Hart, Jon Ivy, John Davison Ng and STEVE! If you would like to join them and our other patrons in supporting what we do here, please visit patreon.com/eons and make your contribution. Now, about whatyou want to learn? Leave us a comment and don't forget to go to youtube.com/eons and subscribe.