Tamed
Page 34
My dear Busk,
A hint or two about the names which I have been rubbing up for the Priscan Pithecoid skull, Homo var. calpicus, from Calpe, the ancient name for the Rock of Gibraltar. What say you?
… Walk up! ladies and gentlemen. Walk up! and see Professor Busk’s Grand, Priscan, Pithecoid, Mesocephalous, Prognathous, Agrioblemmatous, Platycnemic, wild Homo calpicus of Gibraltar …
Yours ever, H. Falconer
But Busk wasn’t quick enough off the mark. Just a few months after Busk’s ‘Pithecoid Priscan Man’ was published, geologist William King, of Queen’s College, Galway, got his hands on a cast of the Feldhofer skull. He recognised it as an ancient type of human, but – rather than an archaic sort of Homo sapiens, he thought its peculiarity warranted giving it a new species name. He proposed Homo neanderthalensis, after the German valley. And so it was King, not Busk or Falconer, who became the very first person to name a species of ancient human – and, of course, the name stuck.
Busk moved on to studying extinct hyenas and cave bears. Falconer died in 1865. And the Forbes skull was tucked away on a shelf again, this time in the Royal College of Surgeons. If events had played out a little differently that year in 1864 – if Busk had been a little less cautious, perhaps – we’d be talking about Calpicans today, not Neanderthals.
Since that first discovery and the recognition that other human species had existed, fossils kept turning up, often in unexpected places, and more and more names were added to branches populated by ancient species more closely related to us than to any other apes – together, we are the hominins. There are now more than twenty named species of hominin, including eight species that have existed within the last 2 million years and that are closely related enough to us to include in our own genus, Homo – the humans.
Neanderthals – the first to be named – remain central to discussions about human origins. Thousands of bones have now been found from more than seventy different sites. And what look like characteristic Neanderthal stone tools have been found at hundreds of sites. For a long time, they’ve been thought of as close cousins of ours. They behaved in similar ways to modern humans living at the same time – they knapped stones to make their hunting weapons and to form scrapers and knives, they buried their dead, they collected shells, used pigment and made marks on the walls of caves. These other people, this ‘lost tribe’, coexisted on the planet with modern humans for thousands of years. But then they disappeared. The enduring question has been – did we meet? Were Neanderthals another ancestor, or just close cousins and – ultimately – a dead end on our family tree?
For years, palaeoanthropologists and archaeologists have argued over the fate of Neanderthals – and in particular, whether modern humans and Neanderthals had ever mixed. Some skeletons appeared to show signs of interbreeding, with classic Neanderthal traits appearing in what otherwise looked like a modern human skeleton. But many experts remained unconvinced. The resolution to the question has had to wait until our modern technology developed to a point where it could provide access to the answer. With new technology, we have the ability to extract and sequence DNA from ancient bones. And finally, it seems that it may be possible to answer the question: did our Homo sapiens ancestors interbreed with Homo neanderthalensis? Are we hybrids?
The unfolding story of human origins
The history of research into human origins follows a course with which we’re now well acquainted. The story began to be pieced together from the study of living humans across the world, and during the nineteenth century there was much discussion over whether humanity could be divided into different races, or even separate species, and if so, whether these had separate origins. As ancient, fossilised bones from earlier types of humans and proto-humans were discovered, starting with those skulls from Gibraltar and Germany, but soon including even earlier fossils from Africa, these had to be fitted into the story too. During the twentieth century, the great debate rumbled on: had modern humans, Homo sapiens, arisen from multiple origins – in Africa, Europe and Asia – or a single one?
The multiregional model proposed that we’d evolved from earlier species across a huge geographic range, with widely separated populations across several continents somehow remaining united through gene flow between them. In contrast, the Recent African Origin or ‘Out of Africa’ model proposed precisely what its name suggests: the emergence of Homo sapiens in a much more discrete geographic region, followed by an expansion of our species across the rest of the Old World, eventually reaching the New World.
In 1971, a determined 22-year-old student called Chris Stringer set off in his ancient Morris Minor to drive across Europe, tracking down fossil skulls in museums. He was armed with measuring instruments – protractors and calipers. He’d written to institutions that he knew possessed ancient skulls, and he’d had some letters back. But with others that he’d heard no word from, he just had to take a chance, and hope that he’d be granted access when he arrived. In the end, Stringer covered 5,000 miles and managed to collect measurements from fossil skulls unearthed in Belgium, Germany, what was still Czechoslovakia back then, Austria, Yugoslavia, Greece, Italy, France and Morocco. He returned to Bristol with all his data, which he analysed using a powerful statistical technique – multivariate analysis – which allows many measurements to be compared at once. He was keen to compare Neanderthal skulls with those of early, modern human Europeans, or Cro-Magnons, from around 30,000 years ago. He hoped to be able to answer that burning question: did Neanderthals evolve into Cro-Magnons, or were they a separate species?
What Stringer found, by comparing the measurements of all those fossil skulls, was that Neanderthals certainly looked like a separate, distinct branch on the human family tree – and they appeared to have evolved in Europe. Cro-Magnons, on the other hand, were clearly part of Homo sapiens, modern humans, and seemed to have suddenly arrived in Europe, rather than to have evolved there in situ. Some scientists had by now suggested that modern humans could have interbred with Neanderthals, in the Middle East or Europe, but Chris found no evidence from the fossils of any interbreeding between the two species.
Stringer wrote up his PhD, having made a significant contribution to some of the big questions around human origins, but what he couldn’t tell from the fossils he’d studied was where modern humans had come from – where they’d first evolved. In 1974, Chris got a chance to look at a skull from Omo-Kibish in Ethiopia, which had been discovered by a team led by Richard Leakey, in 1967. At the time, the skull was estimated to date back to around 130,000 years ago. Many scientists back then thought that Homo sapiens, as a species, was only around 60,000 years old. But when Chris looked at the Omo-Kibish skull, it didn’t appear to be from an archaic species. With its small brow ridge and domed, rounded braincase, the skull looked modern – a good candidate for an ancestor of the Cro-Magnons of Europe. And with a date this early, it suggested that our species may have arisen in Africa.
Over the next decade, more evidence accumulated in favour of a single African origin of our species. By 1987, genetics had joined the debate, with a groundbreaking paper published in the prestigious journal Nature. Three geneticists from the University of California – Mark Stoneking, Rebecca Cann and Allan Wilson – examined the mitochondrial DNA of 147 people from all around the world, and they constructed a phylogenetic family tree from the data. The tree was firmly rooted in Africa. Over the next couple of decades, the genetic data began to pile up, taking in whole genomes, from many more living people, and it all seemed to point to an African origin. That continent contained the most genetic diversity – some 85 per cent of the global total, in fact – a good sign that this was our original homeland. The Omo-Kibish skull itself was re-dated, pushing it even earlier, to almost 200,000 years old. Based on the differences in genomes of modern humans, including living people and ancient ancestors, geneticists have suggested an even earlier divergence date of 260,000 years ago. And in the summer of 2017, there was another revelation as h
uman fossils from Jebel Irhoud in Morocco were dated to between 280,000 and 350,000 years ago. There are several skulls from this site and while the shape of the braincase amongst these is archaic – long and low – the faces are small and pulled back under the braincase: a definitive characteristic of modern humans.
So the picture that had emerged over the forty years following Richard Leakey’s discovery of the Omo-Kibish skull, and over the thirty years since the first mitochondrial DNA studies into human origins, was one of a broad origin across Africa and perhaps slightly beyond, though not as extensive as the Old-World-wide base of the original multiregional theory. But then, some time after 100 thousand years ago, modern humans started to spread beyond that homeland around the globe. They spilled out of Africa, firstly into Arabia and from there round the coast of the Indian Ocean until, by around 60,000 years ago, there were modern humans in Australia. Between 50,000 and 40,000 years ago, modern humans spread westwards into Europe.
But our ancestors weren’t the first humans to live in Europe or Asia. Homo erectus, Homo antecessor, Homo heidelbergensis and Homo neanderthalensis had lived there for hundreds of millennia before our ancestors arrived on the scene. These other species had gone extinct before the moderns arrived. But not Neanderthals. Their population may have been dwindling, hit hard by two particularly nasty downturns in climate in the run-up to the peak of the last Ice Age, but they hung on – until their eventual disappearance from the fossil record some 30,000 to 40,000 years ago.
Through the 1990s and into the 2000s, the debate continued about whether there had been any interbreeding of modern humans with Neanderthals. A few fossils were put forward by some palaeoanthropologists as providing evidence of hybridisation still, but most experts in the field remained unconvinced. Although careful dating of fossils suggested that modern humans and Neanderthals had indeed existed at the same time and in the same general area, in the Middle East and in Europe – with potentially thousands of years of overlap – it seemed that the two populations had always stayed separate. Curiously separate, even. Mitochondrial DNA extracted from Neanderthal fossils was distinct from that of modern humans – with a divergence date estimated at around half a million years ago. Early studies of nuclear DNA from Neanderthal chromosomes suggested a similar date for a last common ancestor of modern humans and those archaic Europeans. And there didn’t seem to have been any mixing of the populations after that split.
Then, in 2010, a group of geneticists working at the Max Planck Institute for Evolutionary Anthropology in Leipzig published an astonishing finding. They’d extracted and analysed DNA from fragments of Neanderthal bone dating to more than 40,000 years ago, from a cave in Croatia. This time, they’d looked more comprehensively at the nuclear genome. And they could compare the draft Neanderthal genome they managed to assemble with genomes of living, modern humans. This comparison revealed that some living people – those of broadly Eurasian ancestry – had more in common with Neanderthals than those with largely African ancestry. The most likely explanation for this inconsistency was that the ancestors of some of us had interbred with Neanderthals. It was an incendiary suggestion. Plenty of scientists published counter-arguments. But as more ancient DNA was recovered from fossils, and compared with the DNA of living humans, it became harder and harder to reject the evidence for interbreeding. Just as japonica rice interbred with proto-indica as it travelled west, and the wonderfully plump apples from Kazakhstan had interbred with European wild apples as they spread through Europe, our modern human ancestors had interbred with the indigenous humans of Europe and western Asia: the Neanderthals.
The development of new genetic tools – ways of analysing DNA inside mitochondria (and inside chloroplasts in plants) and then within the chromosomes themselves – has both enabled and constrained our understanding of the past. Or at least, the early studies were constraining. Both mitochondrial and chloroplast DNA provided a simple and single-rooted path back through history: each is inherited only through the maternal line – although informative, in some ways, each is equivalent to a single genetic marker. There was always a very good chance that this view of history would be unrepresentative of the whole picture. It’s based on such a small fraction of the DNA that cells contain. We need to trace back the evolutionary histories of each gene – and not only that, the stretches of DNA between them that impact on how those genes are read and expressed – before we can truly access the wealth of historical knowledge contained in this amazing biomolecular library. The history of genetic analysis itself has forced a particular trajectory on to the way our understanding of the origin of many species has unfolded – our own included.
So here’s what we know now (though some of this will change as more data comes in): our species originated – probably across a vast, connected area – in Africa (and perhaps even extending into western Asia). Although there may have been earlier migrations, one major migration, starting between 100,000 and 50,000 years ago, led to the colonisation of the rest of the globe. And our ancestors definitely interbred with other, archaic human species or populations. So although the idea of a recent African origin for all of modern humankind has stood up, it’s certainly become blurred around the edges.
Those are the headlines. But the detail is even more fascinating.
Rather than having a well-defined centre of origin, our species may have originated in a diffuse, continent-wide manner – something sounding perhaps more multiregional in nature, but not on a global scale. Much like wheat – where the conceptual origin of einkorn and emmer focused in on the Karacadag Mountains of south-eastern Turkey for a while, then embraced the Middle East – the imagined homeland of modern humans narrowed, to a discrete area of Africa, then broadened – to encompass all of Africa, and perhaps even a bit of Asia too. Various evidence – both genomic and palaeontological – has been put forward to argue for an origin of modern humans in eastern, central or southern Africa. But perhaps we don’t need to choose between them. Modern characteristics may have arisen in a mosaic, piecemeal fashion, spread across populations – connected by gene flow – right across Africa, and a little beyond. African DNA contains echoes of a complex history, with traces of ancient migrations across sub-Saharan Africa, deep splits between populations – but also of mixing together. For tens of thousands of years, Homo sapiens was largely restricted to the continent of Africa – but then the population began to expand, and spread.
The latest, most comprehensive genome-wide analyses support a single, major migration out of Africa, between 50,000 and 100,000 years ago, leading to the colonisation of the rest of the globe. After leaving Africa, the pioneers split into one wave headed eastwards, along the coast of the Indian Ocean – eventually reaching south-east Asia and Australia. Another wave headed north and west, into western Asia and Europe. The eastbound colonisers may have met with modern humans whose ancestors had emerged from Africa during an even earlier migration, reaching all the way to Australia and Papua – as it stands, the fossil record of south and south-east Asia is so thin on the ground that it’s impossible to rule out the existence of a very early eastwards migration.
The date of hybridisation with Neanderthals, the long-time European indigenes, is estimated to be between 50,000 and 65,000 years ago – soon after the out-of-Africa dispersal of modern humans began. Non-African people contain a small percentage of Neanderthal DNA – on average, 2 per cent – whereas people of generally African heritage have little to no Neanderthal DNA knocking around in their genomes. Having had my DNA tested, I am myself, apparently, 2.7 per cent Neanderthal. So I’m not ‘pure’ Homo sapiens. (Nobody is. In fact the whole idea of ‘purity’ in species and subspecies turns out to be an illusion, a hangover from the nineteenth century perhaps, that modern genetics has finally put to rest.) East Asian people tend to have a little more Neanderthal DNA than west Asians and Europeans. There are several possible reasons for this. The ancestors of east Asians – after splitting away from the western Eurasian lot – may have
interbred more enthusiastically with Neanderthals. We also know that Neanderthal DNA has, largely, been weakly selected against since it first arrived in modern human genomes. So it’s possible that the ancestors of both western and eastern populations started out with the same amount of mixed-in, introgressed Neanderthal DNA, and then natural selection could have weeded out more from western Eurasian genomes. Lastly, the lower level in the west may be due to a dilution effect – through interbreeding with populations that didn’t have any Neanderthal DNA, possibly from North Africa.
But it’s not just Neanderthals that our modern human forebears were hooking up with. In the genomes of modern-day people from East Asia, Australia and the Melanesian islands in the south-west Pacific, there are traces of interbreeding with yet another archaic population. In Melanesian genomes, 3–6 per cent of DNA comes from another type of ancestor – known only from a finger bone and a couple of teeth from Denisova Cave in Siberia. We don’t know what these people looked like – the fossil evidence is so meagre. But what we do know, from looking at the ancient DNA extracted from that bone and those teeth, is that they were neither modern humans – nor Neanderthals. There’s just not enough in the way of fossil evidence to give these people their own species name, so for now they’re simply known as ‘Denisovans’. The interbreeding between modern humans and Denisovans probably happened in Asia, before Australia and the Pacific Islands were colonised. There’s also evidence for interbreeding with other, archaic – though currently unidentified – species within Africa. Modern African genomes harbour these memories of other ancient humans, even if we don’t yet have any fossil evidence to link up with those genetic ghosts.