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title: What made us human? The fossils redefining our evolutionary origins

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From Neanderthals to Australopithecus and Paranthropus, the more we’ve learned about ancient hominins, the harder it has become to define what a human is.

Is it in the way we live, laugh and love? Or maybe it is our dislike of cheesy clichés? Deep within each of us, there must be something that makes us distinctly human. The trouble is, after centuries of searching, we still haven’t found it. Perhaps that’s because we have been looking in the wrong place.

Ever since researchers began unearthing ancient hominin bones and stone artefacts, their work has held the tantalising promise of identifying the moment long ago when our ancestors made the transition to become human. Two of the most important fossil discoveries in this quest celebrate significant milestones this year. It is 100 years since the very first “almost human” Australopithecus fossil came to light in South Africa, overturning established thinking about our place of origin. And it is 50 years since the most famous Australopithecus of them all – Lucy, also known as “the grandmother of humanity” – emerged from a dusty hillside in Ethiopia. Both fossils led researchers to believe we really could identify humanity’s big bang: the time when a dramatic pulse of evolution saw the emergence of our human genus, Homo.

But today, the story of humanity’s birth has become far more complicated. A string of discoveries over the past two decades suggests the dawn of our genus is harder to pin down than we had thought. So why did it once seem like Lucy and her ilk allowed us to define humanity and pinpoint its emergence? Why do we now find ourselves as far as ever from establishing what, exactly, a human is? And what does all this tell us about ourselves?

A century ago, the world’s greatest scientific minds were confident that humanity’s roots lay in Eurasia. After all, it was in Europe that the first Neanderthal (Homo neanderthalensis) fossils had been discovered in the 1820s, and in Asia that the first remains of an even more ancient human – now known as Homo erectus – were unearthed roughly 60 years later.

Then anthropologist Raymond Dart at the University of the Witwatersrand in South Africa made a discovery that changed everything. Dart had arranged to receive a consignment of fossils from a South African quarry site now known as Taung. In a crate he opened on 28 November 1924, he found a tiny ancient skull with a mix of ape-like and human-like features. He soon concluded that it represented an extinct genus of “pre-human stock”. Dart named the genus Australopithecus and placed the fossil – nicknamed the Taung Child – in the species africanus.

T9669A Raymond Arthur Dart (February 4, 1893 - November 22, 1988) was an Australian anatomist and anthropologist. In 1924, Dart discovered the first Australopithecus africanus fossil, an extinct hominin closely related to humans.

The roughly 2.8-million-year-old Taung Child was important for two reasons. First, it suggested that our evolutionary roots lay in Africa rather than Eurasia, prompting further investigations in South and East Africa and the recovery of more hominin fossils. Second, it gave us an idea of what our pre-Homo ancestors looked like. However, it still wasn’t clear exactly which species of Australopithecus gave rise to Homo, or when this immediate ancestor had lived.

Then along came Lucy. Still arguably the most famous hominin fossil of them all, Lucy is a roughly 40 per cent complete Australopithecus skeleton. “It wasn’t a single bone or part of a skull. There was an image people could identify with and see that this was an individual,” says Donald Johanson at Arizona State University, who unearthed Lucy in the Afar region of Ethiopia almost 50 years to the day after Dart first held the Taung Child (see “Finding Lucy”, below). Just like Dart, Johanson knew instantly that this was a great discovery.

Taung Child (Australopithecus africanus) skull. This fossil skull was found in a limestone quarry in 1924 in South Africa, and described by Raymond Dart in the Journal of Nature in 1925. The Taung Child skull was that of a three year old bipedal ape, the first member of the genus Australopithecus. The skull is about 2.3 million years old. Australopithecus africanus was an extinct hominid species which may be an ancestor of modern humans.

The first thing that made Lucy special was that she wasn’t alone. Johanson and his colleagues unearthed bits and bobs from dozens of ancient hominins during their Ethiopian fieldwork, providing a rare opportunity to study an ancient hominin population, not just an individual. “Those are the studies that lead to major advances in palaeoanthropology,” says Johanson’s colleague Tim White, now at the University of California, Berkeley. Together with the late Yves Coppens, they eventually concluded that all the fossils at the site represented a single species, which they named Australopithecus afarensis.

The second outstanding thing about Lucy was her pedigree. Johanson and his colleagues declared that A. afarensis occupied a key place in our evolutionary tree: here, at last, was the species that gave rise to our genus, Homo. “It was a pretty bold assertion,” says Johanson. Yet it became accepted by many researchers, even though, at 3.2 million years old, Lucy predated the Taung Child. That the older species, afarensis, should be considered the grandmother of humanity rested in large part on Lucy’s anatomy. Dart’s research left no doubt that Australopithecus was more ape-like than Homo. But the Taung Child and other A. africanus individuals had some features of the face and skull indicating that the species had evolved in a direction distinctly unlike that of our own genus. A. afarensis, by contrast, had none of these evolutionary specialisms. It was anatomically more primitive, making it a great starting point from which to evolve a Homo-like body. This was assumed to have happened around 2.9 million years ago, because that is when A. afarensis went extinct.

Australopithecus africanus pelvis (STS-14). This fossil specimen was discovered in 1947, in Sterkfontein, South Africa. The entire specimen consists of the pelvis, part of the vertebral column, and a femur (lower right). It also includes fragments of a rib (not shown). These fossils are thought to be around 2.5 million years old. Studies of this specimen has shown that this species walked upright. A. africanus is one of several extinct species forming an early part of the human evolutionary tree.

This idea had broader implications. Given that A. afarensis was so primitive compared with the species in our genus, it suggested something momentous must have occurred at the dawn of Homo. “We thought there was this huge change from these ape-like australopiths to these tall and elegant human-like creatures striding across the savannah,” says Carol Ward at the University of Missouri.

There was even a good explanation for such a dramatic evolutionary event. For decades before the discovery of Lucy, researchers had uncovered stone tools alongside the remains of ancient hominins. But with A. afarensis, there were none. “And boy, we certainly looked for them,” says Johanson. It seemed that A. afarensis represented a pre-tool stage of our evolution. This suggested that stone tool technology was a Homo innovation, useful for killing and butchering animals and allowing Homo to adopt a meaty diet that contrasted with the plant-rich diet of A. afarensis. “There was this assumption that several things went together in an evolutionary package,” says John Hawks at the University of Wisconsin-Madison. “Stone tools, new diets, smaller teeth and bigger brains.” This set of features defined Homo, and by extension, it also defined the moment our ancestors became human.

However, in the past 20 years, the sharp dividing line between Australopithecus and Homo has begun to blur. In 2019, Jessica Thompson at Yale University and her colleagues brought the evidence together. One key point is that A. afarensis‘s brain, while small compared with ours, was still 30 per cent larger than those of chimps of a similar size, suggesting Lucy’s species had a calorific diet. Furthermore, its hands were surprisingly human-like, perhaps indicating dexterity. This would fit with recent evidence of cut marks on ungulate bones, suggesting these hominins used stone tools to butcher animals around 3.4 million years ago.

Much of this research remains controversial: White, for instance, has argued that the supposed cut marks are scratches left by crocodile teeth. But Thompson thinks that, collectively, the evidence makes a persuasive case that several traits we had thought unique to Homo were seen in Australopithecus. At a symposium this year to mark the 50th anniversary of Lucy’s discovery, she suggested that A. afarensis might have been a scavenger, targeting animal carcasses and using stone tools to smash open the bones to access the nutritious marrow overlooked by flesh-eating carnivores.

Another recent finding also challenges the idea that hunting is what set Homo apart. A study published last year suggests that Paranthropus, a genus of strange, heavily built hominins that evolved at the same time as Homo, also used stone tools to butcher animals. This indicates that there were fewer differences than we thought between the two contemporary genera. In light of all this, researchers now question whether there really was a dramatic transition between Australopithecus and Homo. “We no longer assume there was an evolutionary package,” says Brian Villmoare at the University of Nevada, Las Vegas.

In retrospect, this makes sense. A. afarensis is astonishingly well represented in the fossil record, but its disappearance some 2.9 million years ago coincides with a million-year period in which the hominin fossil record is patchy. By the time things improve, species that looked and behaved more like we do had appeared. It was easy to assume that those anatomical and behavioural changes had all occurred in one dramatic evolutionary event around 2.9 million years ago. In reality, they probably happened gradually throughout that million-year interval, says Villmoare.

So, what does this mean for our attempts to define when humans evolved? One radical idea is that the term “human” should expand to include every hominin that emerged following the split between our ancestral line and chimps some 7 million years ago. Although popular in the mid-20th century, few researchers favour this view today. However, more recently it has been argued that since the average lifespan of a primate genus is between 7 million and 11 million years, even chimps and bonobos should count as human. At the other extreme is the idea that our ancestors became truly human only after our ancestral line diverged from that of Neanderthals around 800,000 years ago. This rests on the assumption that Neanderthals were brutish and unintelligent. However, following recent archaeological and genetic discoveries, many now argue that there were few behavioural differences between our ancestors and them. “There’s a sense in which ‘becoming human’ can no longer be accounted for by events of the past few hundred thousand years,” says Hawks.

Today, ideas about how to define “human” tend to fall into two camps, one based on behaviour, the other on ancestry. In the former is Bernard Wood at the George Washington University in Washington DC. “When we’re searching for the origin of Homo, we are searching for the first evidence of creatures that go about their lives as we do now,” he says. Australopithecus probably didn’t. Even if we accept that A. afarensis butchered animals, it had the long and powerful arms of a hominin that spent a fair amount of time climbing in trees – something that humans, in general, don’t do. Moreover, some species of Homo, including Homo habilis and Homo floresiensis, also had long and powerful arms, leading Wood and his colleague Mark Collard at Simon Fraser University, Canada, to suggest that we reassign them to Australopithecus. They argue that Homo began only once hominins grew longer legs and became confined to the ground – a stage that was reached with the evolution of H. erectus around 2 million years ago.

Researchers in the other camp include Villmoare, who says genera should be based solely on evolutionary grounds. Do this using the conventional hominin family tree, and Homo is actually easy to define. This is because the descendants of A. afarensis ultimately went down two distinct evolutionary paths: Paranthropus and Homo. Villmoare says this fork in the road, which happened roughly 2.9 million years ago, represents the moment that both genera came into existence. Regardless of the way they looked or behaved, all hominins on the first path belong in the Paranthropus genus and all those on the second path belong in Homo. However, this assumes that the conventional hominin family tree is correct – which it might not be. “There have always been uncomfortable questions surrounding the genus Paranthropus,” says Ward. She suspects that it includes hominins that are superficially similar in appearance but not closely related. If so, that convenient fork in the road after A. afarensis becomes less clear, and ancestry isn’t such a simple way to define what counts as human after all.

A century after the discovery of the Taung Child, and 50 years after Lucy, we seem no closer to understanding when or why humans emerged. That is frustrating, but also, paradoxically, a reflection of how much progress has been made. When researchers had just a handful of hominin fossils to work with, it was easy to spot the significant differences between them and to assume that there was a dramatic moment when our ancestors became human. It is because we have better evidence that we now know things are more complicated.

What is clear is that we didn’t suddenly “become human” 2.9 million years ago when A. afarensis died out. That should come as no surprise. Evolution is a continuous process, and the propensity to divide life up into conspicuously distinct species and genera is less a reflection of fact than another human idiosyncrasy. Ultimately, where we draw the line between “us” and “them” isn’t simply a biological question – it is a philosophical one too.

When Donald Johanson took his first field trip to Africa in 1970, most of the best-known hominin fossil sites there contained geological deposits stretching back no further than 2 million years. But humanity’s origins predate this. “I thought ‘there must be other places to look’,” says Johanson, now a palaeoanthropologist at Arizona State University.

He soon met the late Maurice Taieb, a French geologist who had begun exploring Ethiopia’s Afar region. Taieb’s photographs revealed a landscape littered with animal fossils that were more than 3 million years old. “If human fossils could be found at a place like this, it would open up a significant new vista on our origins,” says Johanson.

It turned out that they could. Several hominin fragments had already been unearthed before Johanson discovered the partial skeleton of a small individual on 24 November 1974. “My girlfriend at the time, Pam Alderman, said if you think it’s a female, why don’t we call it Lucy, after Lucy in the Sky with Diamonds?” says Johanson. “I wasn’t that enthusiastic: I thought she deserved a scientific name.” Eventually, she got one, Australopithecus afarensis. But it is as Lucy that she became a global celebrity.