WHO do you think you are? A modern human, descended from a long line of Homo sapiens? A distant relative of those great adventure-seekers who marched out of the cradle of humanity, in Africa, 60,000 years ago? Do you believe that human brains have been getting steadily bigger for millions of years, culminating in the extraordinary machine between your ears?
Think again, because over the past 15 years, almost every part of our story, every assumption about who our ancestors were and where we came from, has been called into question. The new insights have some unsettling implications for how long we have walked the earth, and even who we really are.
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Once upon a time, the human story seemed relatively straightforward (see blue text in timeline, below). It began roughly 5.5 to 6.5 million years ago, somewhere in an east African forest, with a chimpanzee-like ape. Some of its descendants would eventually evolve into modern chimps and bonobos. Others left the forest for the savannah. They learned to walk on two legs and, in doing so, launched our own hominin lineage.
Rewriting human evolution
The last few years have been marked by a string of truly remarkable finds with huge implications for where we really come from. Get the latest news and insight in this special report
By about 4 million years ago, the bipedal apes had given rise to a successful but still primitive group called the australopiths, thought to be our direct ancestors. The most famous of them, dubbed Lucy, was discovered in the mid-1970s and given arch-grandmother status. By 2 million years ago, some of her descendants had grown larger brains and longer legs to become the earliest “true” human species. Homo erectus used its long legs to march out of Africa. Other humans continued to evolve larger brains in an apparently inexorable fashion, with new waves of bigger-brained species migrating out of Africa over the next million years or so, eventually giving rise to the Neanderthals of Eurasia.
Ultimately, however, those early migrant lines were all dead ends. The biggest brains of all evolved in those hominins who stayed in Africa, and they were the ones who gave rise to Homo sapiens.
Until recently, the consensus was that our great march out of Africa began 60,000 years ago and that by 30,000 years ago, for whatever reason, every other contender was extinguished. Only H. sapiens remained – a species with a linear history stretching some 6 million years back into the African jungle.
Or so we thought.
Starting in the early 2000s, a tide of new discoveries began, adding layer upon layer of complexity and confusion. In 2001 and 2002 alone, researchers revealed three newly discovered ancient species, all dating back to a virtually unknown period of human prehistory between 5.8 and 7 million years ago.
Very quickly, Orrorin tugenensis, Ardipithecus ramidus and Sahelanthropus tchadensis pushed a long-held assumption about our evolution to breaking point. Rough genetic calculations had led us to believe our line split from the chimp lineage between 6.5 and 5.5 million years ago. But Orrorin, Ardipithecus and Sahelanthropus looked more like us than modern chimps do, despite predating the presumed split – suggesting our lineage might be at least half a million years older than we thought.
At first, geneticists made grumpy noises claiming the bone studies were wrong, but a decade later, even they began questioning their assumptions. In 2012, revised ideas about how quickly genetic differences accumulate in our DNA forced a reassessment. Its conclusion: the human-chimp split could have occurred between 7 and 13 million years ago.
Not so chimp-like
Today, there is no longer a clear consensus on how long hominins have walked the earth. Many are sticking with the old assumption, but others are willing to consider the possibility that our lineage is almost twice as old, implying there are plenty of missing chapters to our story still waiting to be uncovered.
The struggles don’t end there. The idea that our four-legged ancestors abandoned the forests, perhaps because of a change in climate conditions, and then adapted to walk on two legs is one of the oldest in human evolution textbooks. Known as the savannah hypothesis, it was first proposed by Jean-Baptiste Lamarck in 1809. Exactly 200 years later, an exquisite, exceptionally preserved 4.4-million-year-old skeleton was unveiled to the world, challenging that hypothesis.
“Ardi”, a member of A. ramidus, is a jewel in the hominin fossil record. She is all the more important because of the number of key assumptions she casts doubt on. Ardi didn’t have a chimp’s adaptations for swinging below branches or knuckle-walking, suggesting chimps gained these features relatively recently. In other words, the ape that gave rise to chimps and humans may not have been chimp-like after all.
And contrary to Lamarck’s hypothesis, her feet, legs and spine clearly belonged to a creature that was reasonably comfortable walking upright. Yet, according to her discoverers, Ardi lived in a wooded environment. This suggests that hominins began walking on two legs before they left the forests, not after – directly contradicting the savannah hypothesis.
Although not everyone is convinced that Ardi was a forest-dweller, other lines of evidence also suggest we have had the upright walking story back to front all these years. Susannah Thorpe at the University of Birmingham, UK, studies orangutans in their natural environment and has found that they stand on two legs to walk along branches, which gives them better access to fruit. In fact, all living species of great ape will occasionally walk on two legs as they move around the forest canopy. It would almost be odd if our own ancestors had not.
Whether before or after standing on two legs, at some stage our ancestors must have come down from the trees. We can depend on that, at least. Entering the 21st century, we knew of just one group that fitted the transition stage: the australopiths, a group of ape-like bipedal hominins, known from fossils found largely in east and south Africa and dating to between 4.2 and 1.2 million years ago. They lived in the right place at the right time to have evolved into humans just before 2 million years ago. Lucy would have showed up in the middle of that period, 3.2 million years ago. Since her discovery, she has served as a reassuring foundation stone on which to build the rest of our hominin family tree, a direct ancestor who lived in east Africa’s Rift Valley.
Then, in 2001, researchers unveiled a 3.5-million-year-old skull discovered in Kenya. The skull should have belonged to Lucy’s species, A. afarensis, the only hominin species thought to be living in east Africa at the time. But its face didn’t fit. It was so flat that it could barely be considered an australopith, says Fred Spoor at University College London, who analysed the skull. He and his colleagues, including Meave Leakey at Stony Brook University in New York, gave it a new name: Kenyanthropus platyops.
On the face of it, the suggestion that Lucy’s species shared east Africa with a completely different type of hominin seemed only of marginal interest. But within a few years, the potential significance of Kenyanthropus was beginning to grow. After comparing the skull’s features with those of other hominin species, some researchers dared suggest that K. platyops was more closely related to us than any australopithecus species. The conclusion pushed Lucy on to a completely different branch of the family tree, robbing her of her arch-grandmother position.
If that wasn’t confusing enough, other researchers were making a similar attack from a different direction. The discoverers of O. tugenensis, the 6-million-year-old hominin found in 2001, also concluded that its anatomy was more human-like than that of australopiths, making it more likely to be our direct ancestor than Lucy or any of her kin.
Most of the research community remains unconvinced by these ideas, says Spoor, and a recent announcement that a human-like jawbone 2.8 million years old had been discovered in Ethiopia once more shored up Lucy’s position. “In many respects it’s an ideal transitional fossil between A. afarensis and earliest Homo,” says Spoor.
Even so, Lucy’s status as our direct ancestor has been formally challenged, twice, and Spoor says it’s not inconceivable that the strength of these or other challenges will grow. “We have to work with what we have and be prepared to change our minds if necessary.”
Tiny brains and alien hobbits
Intriguingly, in 2015, a team announced the discovery of the oldest known stone tools. The 3.3-million-year-old artefacts were found in essentially the same deposits as Kenyanthropus. “By all reasonable logic Kenyanthropus would be the tool-maker,” says Spoor. Perhaps that hints at a tool-making connection between Kenyanthropus and early humans – although there is circumstantial evidence that some australopiths used stone tools too. In any event, determining which hominins evolved into humans is no longer as clear-cut as it once was.
Other important parts of the human evolution narrative were untouched by these discoveries, in particular, the “out of Africa” story. This idea assumes that the only hominins to leave Africa were big-brained humans with long legs ideally suited for long-distance travel.
But discoveries further afield have begun to chip away even at this core idea. First came news, in 2002, of a 1.75-million-year-old human skull that would have housed a brain of no more than 600 cubic centimetres, about half the size of modern human brains. Such a fossil wouldn’t be an unusual find in east Africa, but this one turned up at Dmanisi in Georgia, in the Caucasus region. Clearly, small-brained hominins had left Africa.
In other respects, the Dmanisi skull and several others found at the site did not threaten the standard narrative. The Dmanisi hominins do seem to be early humans – perhaps unusually small-brained versions of H. erectus, conventionally regarded as the first hominin to leave Africa.
A discovery in 2003 would ultimately prove far more problematic. That year, researchers working on the Indonesian island of Flores found yet another bizarre skeleton. It had the small brain and small body of an early African hominin, from around 2 to 3 million years ago. To make matters worse, it seemed to have been alive just a few tens of thousands of years ago in a region thought to be home only to “true” long-limbed and large-brained humans. The team named the peculiar species Homo floresiensis, better known by its nickname: the hobbit.
“I said in 2004 that I would have been less surprised if they had found an alien spacecraft on Flores than H. floresiensis,” says Peter Brown at the Australian National University, who led the analysis of the remains. The primitive-looking skeleton was, and still is, “out of place and out of time”.
There’s still no agreement on the hobbit’s significance, but one leading idea is that it is evidence of a very early migration out of Africa involving prehuman australopith-like hominins. In fact, the entire out-of-Africa narrative is in flux, with genetic and fossil evidence suggesting that even the once widely held opinion that our species left Africa 60,000 years ago is hopelessly wrong. Some lines of evidence suggest H. sapiens may have reached China as early as 100,000 years ago.
The hobbit was just one bizarre hominin, and could reasonably be discounted as a simple anomaly. But within little more than a decade of its discovery, two more weird misfits had come to light, both in South Africa.
Australopithecus sediba and Homo naledi are quite unlike any hominin discovered before, says Lee Berger at the University of Witwatersrand in South Africa, who led the analysis of both. Their skeletons seem almost cobbled together from different parts of unrelated hominins. Significantly, the mishmash of features in the A. sediba skeleton, unveiled in 2010, is very different from those in the H. naledi skeleton, unveiled in 2015.
A. sediba‘s teeth, jaws and hands were human-like while its feet were ape-like. H. naledi, meanwhile, combined australopith-like hips with the skull of an early “true” human and feet that were almost indistinguishable from our own.
“Our human history made great sense – right up to the moment it didn’t”
No other ancient species seems quite as strange – but, as Berger points out, very few other ancient hominins are preserved in so much detail. Perhaps that’s just an interesting coincidence. Or perhaps, he says, it’s a sign that we have oversimplified our understanding of hominin evolution.
We tend to assume that ape-like species gradually morphed into human-like ones over millions of years. In reality, Berger thinks, there may have been a variety of evolutionary branches, each developing unique suites of advanced human-like features and retaining a distinct array of primitive ape-like ones. “We were trying to tell the story too early, on too little evidence,” says Berger. “It made great sense right up until the moment it didn’t.”
Earlier this year, Berger announced the age of the H. naledi remains. They are just 236,000 to 335,000 years old. Weeks later, news broke that 300,000-year-old fossils from Morocco might belong to early members of H. sapiens. If correct, the fossil extends our species’ history by a whopping 100,000 years.
H. naledi‘s relatively young age is also a striking example of how complex and confusing the human evolutionary tree might really be. Human brains didn’t grow and grow for millennia, with smaller-brained species falling to the wayside of the gradual evolutionary road. Instead, our species occupied an African landscape that was also home to humans with brains half the size of theirs.
We can only speculate on how (or whether) the small-brained H. naledi interacted with the earliest H. sapiens. Tantalising but controversial evidence from Berger’s team suggests that H. naledi intentionally disposed of its dead – perhaps a sign that even “primitive” hominins could behave in an apparently sophisticated way (see “Not so special after all“).
Another independent line of evidence suggests that different behaviour was not necessarily a barrier to inter-species interactions.
In the late 1990s, geneticists began to show an interest in archaeological remains. Advances in technology allowed them to sequence a small chunk of mitochondrial DNA (mtDNA) from an ancient Neanderthal bone. The sequence was clearly distinct from H. sapiens, suggesting that Neanderthals had gone extinct without interbreeding (“admixing”) much with our species.
But mtDNA is unusual. Unlike the nuclear DNA responsible for the bulk of human genetics, it passes intact from a mother to her children and doesn’t mix with the father’s genes. “Mitochondrial DNA is the worst DNA you can choose to look at admixture,” says Johannes Krause at the University of Tübingen in Germany.
By 2010, a very different picture was emerging. Further advances in technology meant geneticists such as Krause could piece together a full nuclear genome from Neanderthal bones. It showed subtle but distinct evidence that Neanderthals had interbred with our species after all. The behavioural differences between humans and Neanderthals were evidently not enough to preclude the occasional tryst.
Arguably, this wasn’t the biggest genetics announcement of the year. In their searches, Krause and his colleagues had examined genetic material extracted from a supposed Neanderthal bone fragment unearthed in a Siberian cave in 2008. To everyone’s surprise, the DNA in the bone wasn’t Neanderthal. It came from a related but distinct and entirely new hominin group, now dubbed the Denisovans.
To this day, the Denisovans remain enigmatic. All that we have of them are one finger bone and three teeth from a single cave. We don’t know what they looked like, although H. sapiens considered them human enough to interbreed with them: a Denisovan nuclear genome sequence published in 2010 showed clear evidence of sex with our species. The DNA work also shows that they once lived all across East Asia. So where are their remains?
Slap and tickle
Fast-forward to 2017, and the interbreeding story has become more complex than anyone could have imagined in 2000. Krause reels off the list. “Neanderthals interbred with H. sapiens. Neanderthals interbred with Denisovans. Denisovans interbred with H. sapiens. Something else that we don’t even have a name for interbred with Denisovans – that could be some sort of H. erectus-like group…”
Although weird bones have done their bit to question our human history, it’s the DNA inside them that may have done the most to shake up our evolutionary tree. With evidence of so much ancient interbreeding, it becomes far more complicated to decide where to draw lines between the different groups, or even if any lines are justified.
“How do you even define the human species now?” says Krause. “It’s not an easy discussion.” Most of us alive today carry inside our cells at least some DNA from a species that last saw the light of day tens of thousands of years ago. And we all carry different bits – to the extent that if you could add them all up, Krause says you could reconstitute something like one-third of the Neanderthal genome and 90 per cent of the Denisovan genome. With this knowledge, can we even say that these species are truly extinct? Pushing the idea one step further, if most living humans are a mishmash of H. sapiens DNA with a smattering from other species, is there such a thing as a “true” H. sapiens?
Having dug ourselves into this philosophically troubling hole, there’s probably only one way to find our way out again: keep digging for fossils and probe them for more DNA.
Not so special after all
Our species, Homo sapiens, is special. We have achieved things beyond the capacities of all others in our family tree. Even with their wanderlust, the ancient humans that came before us probably never made it to the Americas, let alone reaching for the moon, of course. Ancient human species never learned to write, or compose symphonies, nor did they develop the scientific nous to explore their own evolutionary roots.
But the distinction between our species and those that went before may not be quite as stark as we once thought. In 2014, for instance, researchers found a zigzag that had been etched in a shell 500,000 years ago. We had thought we were the only species to produce abstract symbols, yet here was H. erectus doing so more than 200,000 years before H. sapiens even evolved.
Researchers are also becoming increasingly convinced that Neanderthals had advanced behaviour, like using watercraft to reach islands or exploiting simple chemistry to start fires. Some suggest they carved a hashtag sign on to a rock in Gibraltar. In a cave in France, they built mysterious stone circles out of stalagmites (pictured above). Were these symbols too?
And then there’s H. naledi, with a brain less than half the size of our own. According to the team that excavated its remains, H. naledi might have deliberately disposed of its dead in deep, inaccessible cave chambers. Such behaviour seems strikingly modern, not at all the sort of thing expected of a hominin with a brain only marginally larger than a chimp’s.
H. sapiens still stands out as a truly exceptional hominin – but the deeper we dig, the more we see echoes of our sophisticated behaviour in some of our ancient relatives.
This article appeared in print under the headline “Losing the plot”
