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Bigger brains
Famously, our species is known as Homo sapiens. But we are not the only species to have borne the name Homo – not by a long shot. The first Homo species evolved more than 2.8 million years ago, long before we came along, and they are some of the most remarkable creatures ever to have lived. For one thing, they became some of the most widespread animals on Earth.
If there is one word to describe the fossil record of the Homo genus, it is ‘confusing’. A host of species of Homo has been described, often on the basis of incomplete or even fragmentary skeletal remains. Vigorous arguments rage over which of them are true species and which simply represent variation within a species. It is not uncommon to find the same fossil grouped into three different species by different researchers. But if we take a step back, we can reduce the first Homo to three main species: Homo habilis, H. erectus and H. heidelbergensis. Between them, these three species account for a big chunk of the story.
 
Beginnings
The oldest known species of Homo is Homo habilis, which was confined to Africa. The year 2015 saw the discovery of the oldest known fossil belonging to this species. Unearthed in Ethiopia, the broken jaw with greying teeth suggests that the Homo lineage existed up to 400,000 years earlier than previously thought. The fragment dates from around 2.8 million years ago, and is by far the most ancient specimen to bear the Homo signature. Previously, the earliest such fossil was one thought to be up to 2.4 million years old.
The fossil has a mixture of traits. It may pinpoint the time when humans began their transition from primitive, ape-like Australopithecus to the big-brained conqueror of the world. Geological evidence showed that the jaw’s owner lived just after a major climate shift in the region. Forests and waterways rapidly gave way to arid savannah, leaving only the occasional crocodile-filled lake. Except for the sabre-toothed big cats that once roamed these parts, the environment ended up looking much as it does today. The pressure to adapt to this new world may have jump-started our evolution into what we see looking back at us in the mirror today.
The emerging Homo species probably began eating more meat and using better tools – a change reflected in a more delicate jaw unearthed in 2013. After all, if you had a nice sharp stone to cut with, there was no need for a mouth built for tearing food to shreds.
Handy man
Figure 4.1 shows a reconstruction of the skull of one of the first known members of the human genus, Homo habilis, which means ‘handy man’, from about 1.8 million years ago.
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FIGURE 4.1   A reconstructed Homo habilis skull based on the bones of two specimens, one from modern-day Tanzania and one from Kenya
The original fossil, from Tanzania, which was first reported in 1964, is incomplete. It consists of just a few distorted fragments. But in 2015 Fred Spoor at University College London led a team to create a computer reconstruction that realigned the fragments and filled in the missing parts. This made it possible to compare the skull with other fossils from what was a critical time for early human evolution.
The digitally reconstructed skull shows that handy man shared some features with H. erectus, but in other ways resembled Australopithecus afarensis (Lucy’s species), which lived some 3.2 million years ago.
Man of grass
One of the stories that is perpetually told about human evolution is that our ancestors ‘came down from the trees’ and moved out ‘on to open grassland’. The move down from the trees actually predates the Homo genus: Australopithecus and even the older Ardipithecus clearly spent plenty of time on the ground. But the move to grassland came later.
Still, by 2 million years ago humans were living and thriving on open grassland in Africa, making stone tools and using them to butcher zebra and other animals. That’s according to powerful evidence from artefacts found at Kanjera South, an archaeological site in south-western Kenya.
There is no clear evidence of any hominin being associated with, or foraging in, open grassland prior to this time. Other earlier hominins that have been found in the geological record – such as Ardipithecus ramidus and Australopithecus afarensis – lived either in dense forest or in a mosaic of woodland, shrub and grasses.
The Kanjera South site offers a glimpse into the lives of our ancestors as they were starting to adapt to life on the plains. The site is a grassland setting, dominated by grass-eating animals. Tests show that the site was over 75 per cent grassland 2 million years ago. The wider area was teeming with zebras, antelope and other grazers, and all the animals carried the same telltale chemical signal suggesting that they were eating grass.
Say yes to meat
The dawn of the genus Homo, around 2.5 million years ago, was a watershed for another reason. This seems to have been the time when our ancestors evolved beyond their vegetarian roots and became meat eaters.
In 1999 researchers found cut marks on animal bones dated at around 2.5 million years old. But no one could be sure that they were made by meat-eating hominids, because none appeared to have suitable teeth. However, a 2013 study revealed that the first members of Homo had much sharper teeth than their most likely immediate ancestor, Australopithecus afarensis, the species that produced Lucy.
Eating meat requires teeth adapted more to cutting than to grinding. The ability to cut is determined by the slope of the cusps, or crests. Steeper crests allow a creature to consume tougher foods. The crests of teeth from early Homo skeletons are steeper than those of gorillas, which consume foods as tough as leaves and stems, but not meat. The crests of teeth from A. afarensis are not only shallower than those of early Homo but also shallower than those of chimpanzees, which consume mostly soft foods such as ripe fruit, and almost no meat. In other words, early Homo had teeth adapted to tougher food than A. afarensis or chimpanzees. The obvious candidate is meat.
This finding is fairly uncontroversial. But some anthropologists have advanced a much more radical idea: that early Homo were not just eating meat but also cooking their food. That would imply that they had discovered how to control fire.
The idea that the invention of cooking fundamentally influenced our evolutionary past was given a boost in 2003 by a study of modern diets. A team of anthropologists concluded that this newfound culinary talent was the only way to explain the huge change in our evolution 1.9 million years ago, when Homo erectus appeared. This was a more human-like species that was so successful that it spread out of Africa all the way to Java, Indonesia.
H. erectus was 60 per cent larger than its predecessors, and sported the largest increase in human brain size ever seen. Some experts believe that this growth spurt was fuelled by protein derived from eating raw meat. But anthropologist Richard Wrangham of Harvard University in Cambridge, Massachusetts, has long argued that it was triggered by cooking plant food, such as roots and tubers.
The heat of cooking smashes open cells and breaks down indigestible fibre into energy-giving carbohydrates. The advent of cooking would therefore account for H. erectus having a shorter gut and smaller teeth, and explain why early humans became more sociable as they brought food back to a central cooking area.
In support of this idea, Wrangham found that people need to eat twice as much raw food as cooked food to gain the same energy from a vegetarian diet, and 50 per cent more if their diet includes raw meat as well as plants. From a study of people in Germany who ate a raw food diet, Wrangham calculated that a person eating uncooked, vegetarian food would have to consume around 9 per cent of their body weight every day to get enough calories to maintain a leisurely modern Western lifestyle. That’s more food than the average American eats on Thanksgiving Day.
The origin of cooking: an interview with Richard Wrangham
Cooking is what allowed us to become human, says anthropologist Richard Wrangham, author of Catching Fire: How Cooking Made Us Human (2010).
What was the central mystery of human evolution that you were trying to solve?
I was sitting next to the fire in my living room and I started asking the question, when did our ancestors last live without fire? Out of this came a paradox: it seemed to me that no human with our body form could have lived without it.
Why can’t a human exist on the same diet as a chimpanzee?
A chimpanzee’s diet is like eating crab apples and rose hips. Just go into the woods and find some fruits, and see if you can come back with a full stomach. The answer is you can’t. The big difficulty is that the nutrient density is not very high. This is problematic for humans because we have a very small gut, about 60 per cent of the volume it would be if we were one of the other great apes. We don’t have enough intestine to keep low-quality food in our gut long enough to digest it.
So cooking provided some kind of a watershed for humans to split from our chimp-like ancestors?
Yes; I believe the point at which our bodies show adaptation to cooking is 1.9 million years ago. The evidence is in the changes that took place when we evolved from ancestors that were like chimpanzees but were already standing upright. Cooking led to increased energy intake.
What was the result of having more energy?
Maximizing energy from food allowed us to lose a third of the large intestine and significantly expand our brain size. It affected our brain because humans were social and there was a premium on being as intelligent as possible in order to outsmart your opponents in competition, ultimately for mates.
The cooking row
The key stumbling block for the theory that our early ancestors cooked their food is the lack of convincing evidence that hominins could control fire more than a million years ago. This problem only got worse in 2011, when evidence emerged that humans only began controlling fire very recently.
A review of supposed archaeological hearths in Europe suggested that the oldest date was just 400,000 years ago. The finding suggests that humans expanded into cold northern climates without the warmth of fire – and that cooking was not the evolutionary trigger that boosted our brain size.
Many of the ‘smoking guns’ for prehistoric fire use – charred bone fragments or chunks of charcoal – do not necessarily imply that early humans could control fire. Our opportunistic ancestors may simply have exploited the occasional wildfires triggered by lightning, for example.
To try to pin down the earliest evidence of controlled fire use, researchers re-examined the data from more than a hundred European sites. They were looking for evidence of fires that were unlikely to have occurred naturally – those in caves, for example – and for clues that fire had been used in a controlled way. These include activities such as making pitch: some early hominins made this sticky substance by burning birch bark and using it to glue pieces of flint to wooden handles, to make stone tools easier to use.
The earliest European hearths date back between 300,000 and 400,000 years, the researchers concluded. Although the study investigated only European sites, evidence of controlled fire use at a number of other sites is also up for debate. The Swartkrans site in South Africa is believed by some to contain 1.6-million-year-old evidence, in the form of hundreds of charred bones. But that might just have been sporadic natural fires that people used to their advantage. In fact, just one site earlier than the 400,000-year mark has strong evidence of controlled fire use: the 780,000-year-old Gesher Benot Ya’aqov site in Israel, where charred flints, seeds and stone tools have been found.
The conclusion questioned Wrangham’s hypothesis that an increase in human brain size was tied to the invention of cooking. However, the story does not end there. Another study, from 2012, presented evidence that the control of fire came at least a million years ago.
There are no obvious hearths in South Africa’s Wonderwerk Cave. So, instead, researchers used microscopic analysis to study the sediments on the cave floor. They found evidence of ash and traces of burnt bone in layers that formed a million years ago. The burnt remains are 30 metres from the present entrance to the cave, so they are unlikely to represent the action of wildfires. It is more likely that hominins – probably Homo erectus – carried fire into the cave. The burnt bone fragments – including bits of tortoise bone – suggest, but do not prove, that H. erectus was cooking food.
However, this is hardly final proof of Wrangham’s cooking hypothesis. The tiny traces of fire in the cave stand in contrast to the extensive ash deposits from fire found in much later sites of human occupation. This suggests that H. erectus was not using fire regularly, or routinely cooking food – despite its small teeth and large brain.
Homo erectus leaves Africa
Whatever the truth of the claim that early Homo species worked out how to cook food, one of these species clearly did achieve something that none of their ancestors and relatives did. From Orrorin through to Australopithecus, all the hominins were confined to Africa. There are no fossils of them from anywhere else.
But all that changed with the rise of Homo erectus. The first fossil of this species was not found in Africa at all, but on Java in Indonesia: hence its early nickname ‘Java Man’. Evidently, some H. erectus managed to migrate out of Africa and spread throughout much of Europe and Asia. Yet, compared with modern humans, H. erectus had a small brain and could make only the simplest tools. This suggests that it did not take any great intelligence for them to go global.
Some of the best fossils of H. erectus outside Africa come from a site in Dmanisi, Georgia. They include an entire skull from an H. erectus that lived 1.8 million years ago – the earliest completely preserved specimen ever found.
An even earlier exit from Africa?
There is tentative evidence that hominins left Africa even sooner than has been thought, perhaps even before H. erectus evolved – but this remains a minority view.
In 2016 scientists claimed that humans were living in India 2.6 million years ago, based on an analysis of stone tools and three cow bones with cut marks. Researchers found the artefacts on the Siwalik Hills about 300 kilometres north of New Delhi, India. There, tectonic activity has exposed an outcrop of bedrock dating back at least 2.6 million years. The bones and tools were found lying on the surface. The team’s examination of the cut marks on the bones suggested that they were made with a stone tool. On this basis, they claimed that hominins lived there 2.6 million years ago.
Taken at face value, the finds suggest that our Homo genus had migrated into Asia much earlier. Another possibility is that the earlier ape-like Australopithecus lived in Asia as well as Africa. But the evidence is weak. In particular, it is problematic that the stone tools and bones were found on the surface rather than in a dateable rock layer.
A link to the Neanderthals
The third crucial Homo species was H. heidelbergensis. This species lived relatively recently, probably from 700,000 to 200,000 years ago. It’s generally thought that they evolved from older H. erectus populations, and that they gave rise to modern humans, as well as our cousins the Neanderthals and Denisovans. This species is therefore thought to have been a crucial stepping stone – the ancestor of modern humans as well as our close relatives the Neanderthals (see Chapter 5).
This story makes a certain amount of intuitive sense, because there is evidence that H. heidelbergensis were remarkably advanced for their time. For instance, in 2012 archaeologists found the oldest evidence of stone-tipped spears. The discovery in South Africa suggested that it was neither our species nor Neanderthals that pioneered the use of such spears, but H. heidelbergensis.
In line with this, there is a 400,000-year-old site near Schöningen, Germany, where wooden spears have been found associated with the remains of 19 horses. This seems to suggest that H. heidelbergensis mounted a carefully planned ambush there.
There is also intriguing evidence that H. heidelbergensis cared for invalids. In 2010 archaeologists described the most elderly ancient human ever found. He was an H. heidelbergensis that lived 500,000 years ago, and was about 45 years old when he died. He has been named ‘Elvis’, after his pelvis and lower backbone were uncovered in the Atapuerca Mountains, northern Spain. Elvis was too old to hunt and suffered terrible lower back pain. His spine was bent forward. To keep an upright posture he may even have used a cane, just as elderly people do today.
The fact that Elvis was so infirm suggests that his contemporaries must have looked after him. He could not have been physically active, but he may have had valuable knowledge that he shared with other members of the group that helped them survive. In line with this, in 2009 the same team reported evidence from Atapuerca that a 12-year-old child with skull malformations was cared for by the same group.
Ancestry in doubt
In recent years it has become possible to read DNA from preserved hominin bones, and a series of such studies has thrown the conventional H. heidelbergensis story into doubt.
In 2016 scientists described the oldest DNA from the nucleus of a human cell to be sequenced so far. The 430,000-year-old DNA came from mysterious early human fossils found in the Sima de los Huesos, or ‘pit of bones’, in the Atapuerca Mountains. The DNA revealed Neanderthals in the making. The Sima fossils look as if they come from ancestors of the Neanderthals – who evolved some 100,000 years later. But, confusingly, a 2013 study found that their mitochondrial DNA is more similar to that of Denisovans (see Chapter 5), who also lived later – thousands of kilometres away, in southern Siberia.
So who were the Sima people and how are they related to us? To find out, geneticists pieced together parts of the Sima hominin’s nuclear DNA from samples taken from a tooth and a thighbone. The results suggested that they are more closely related to ancestors of Neanderthals than to those of Denisovans – meaning that the two groups must have diverged by 430,000 years ago. This is much earlier than the geneticists had expected.
It also alters our own timeline. We know that Denisovans and Neanderthals shared a common ancestor that had split from our modern human lineage. In the light of the new nuclear DNA evidence, this split might have happened as early as 765,000 years ago.
Conventional thinking is that modern humans, Neanderthals and Denisovans all evolved from H. heidelbergensis. However, H. heidelbergensis did not evolve until 700,000 years ago – potentially 65,000 years after the split between modern humans and the Neanderthals and Denisovans. Instead, another, obscure species called Homo antecessor might now be in the frame as our common ancestor. This species first appeared more than a million years ago.
Still, even if H. heidelbergensis was not our direct ancestor, it seems likely that it was quite closely related to it.
The discovery of Homo naledi
There is one more Homo species to discuss – the mysterious Homo naledi. H. naledi was revealed only in 2015, and research is proceeding apace – so fast, in fact, that this section of the book, more than any other, is likely to be out of date within a few years.
The story begins in the Rising Star cave system in South Africa. On 13 September 2013 two cavers, Steven Tucker and Rick Hunter, made their way down into the maze of dark passages. The pair were hoping to find tunnels that no human had ever set foot in before. Having crept up a narrow ridge known as the Dragon’s Back, with 15-metre drops on either side, Hunter and Tucker arrived in a chamber thought to be a dead end. But, peering down, they discovered a narrow chute that led into another chamber.
Tucker went in first. Twelve metres down the chute, he emerged through the roof of another chamber and climbed down to the floor. The room was barely 3 metres wide. A narrow passage leading out of this chamber and on to another was just wide enough to pass through, so he called Hunter to join him. The first thing Tucker saw when they shuffled through into the next chamber was yet another passageway leading out of it. The bones came second. They were sticking out of the cave floor.
Palaeoanthropologist Lee Berger of the University of the Witwatersrand (the discoverer of A. sediba, discussed in Chapter 3) had been asking caving clubs to get their members to look out for fossils. So when Tucker and Hunter spotted a jawbone with what looked like human teeth, they snapped a few pictures before moving on.
Three days later, the pair were in Berger’s office in Johannesburg. When he saw the pictures, Berger’s jaw dropped. He immediately knew that the bones did not belong to Homo sapiens. Before the week was out, Berger went to see the cave. He couldn’t fit down the chute, so he sent his teenage son into what is now called the Dinaledi Chamber with Hunter and Tucker. When Matthew Berger saw the bones, his hands began to shake. It was minutes before he could steady them enough to take pictures.
Just days later, on 6 October, Lee Berger posted an appeal on Facebook for palaeoanthropologists – preferably those with caving skills, and ideally with small frames. People were recruited within days – and the expedition was on.
H. naledi unveiled
Two years later, in September 2015, Berger and his colleagues published their first trove of results. The remains belonged to a previously unknown early species of our own genus, Homo – and they named it Homo naledi.
The species had a unique mix of characteristics. Look at its pelvis or shoulders and you would think it was an ape-like Australopithecus from 3–4 million years ago. But look at its foot and you could think it belonged to our species, which appeared within the last 500,000 years. Its skull, though, made it clear that the brain was less than half the size of ours and more like that of some species of Homo that lived about 2 million years ago (see Figure 4.2).
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FIGURE 4.2   A replica of the skull of ‘Neo’, an adult Homo naledi with a remarkably complete skeleton
The team refers to the fossils’ mixture of features as ‘anatomical mosaic’. We have previously seen such a mosaic in Australopithecus sediba, the 2-million-year-old hominin that Berger excavated in 2008. Although it was just about possible to dismiss A. sediba, with its assortment of ancient and modern features, as a quirk of human evolution, the new find hints that such ‘mosaicism’ is not the exception in early humans but the rule.
This has implications for how we interpret other early human fossil finds representing the transition from Australopithecus to Homo. These fossils generally amount to just a few fragments rather than complete skeletons, and that might not be enough to tell us where they fit.
There is another possible conclusion to draw from the find. The sheer number of bones and their location hint at something astonishing: the bodies they belonged to appear to have been left deliberately in the cave. This has never been seen before in such a primitive human, and could have big implications for understanding the origins of modern human behaviour.
Besides a few rodent fossils and the remains of an owl that probably fell into the Dinaledi chamber by mistake, there are no other vertebrate species present. How so? Only one scenario works, the researchers argued: H. naledi deliberately disposed of its dead in the chamber. Perhaps the bodies were gently dropped down the shaft that researchers squeezed through to recover the bones.
There are precedents for this. At Sima de los Huesos in Spain, 28 hominin skeletons were recovered from a deep pit. But those hominins were big-brained – they looked and behaved rather like us. H. naledi had a brain less than half the size of ours.
The age of naledi
Two years later, in May 2017, Berger and his team published a slew of new results. The team had recovered 130 additional hominin bones and teeth from a second chamber in Rising Star, named the Lesedi Chamber. The additional H. naledi remains belong to at least three individuals, and many of the bones and teeth belong to a single, remarkably complete adult skeleton, dubbed Neo.
Perhaps more significantly, for the first time the team had worked out the age of the H. naledi remains in the Dinaledi Chamber: they were between 236,000 and 335,000 years old. This age range is significant. It puts H. naledi on the South African landscape not long before our species had begun to appear elsewhere in Africa – and long after small-brained hominins were thought to have vanished from the continent.
The age of the H. naledi bones also falls in a time period with a generally poor hominin fossil record. We know that several species of hominin apparently coexisted in Africa more than 2 million years ago, and that several species seem to have coexisted across Eurasia in the past 100,000 years or so. Now it seems that there was also diversity around the 250,000-year mark.
However, it is less clear where H. naledi fits into the human family tree. A full evolutionary analysis might conclude from those modern hands and feet that H. naledi branched off from other humans relatively recently. This would mean that it originated recently and then evolved to look more primitive due to its isolation.
For instance, southern Africa might have been relatively isolated from the rest of the continent, and H. naledi’s lineage might have had comparatively little competition from other humans. This could have relaxed the pressure to grow and maintain a large brain. If the skeleton no longer had to bear the weight of a large and heavy skull, features like the hips and shoulders might have reverted to become more like those of a small-brained hominin.
But others are reasonably sure that H. naledi is genuinely an early human – albeit one that survived until astonishingly recently. It could be the most primitive early Homo ever discovered. The species might have evolved more than 2 million years ago, as one of the earliest ‘true’ humans, and then survived, unchanged, for hundreds of thousands of years. H. naledi might be a kind of ‘living fossil’, a human version of the coelacanth – a primitive fish with ancestors that first appeared 400 million years ago but that is still found in oceans today. In other words, species of evolutionarily primitive humans might, in some circumstances, be able to survive for hundreds of thousands of years.
This would flip the aforementioned model on its head. Rather than seeing southern Africa as an isolated evolutionary cul-de-sac, perhaps it was actually the powerhouse of human evolution: the region where many human species (potentially including ours) first appeared.
However, this is speculation. There is one more question to ask: what ultimately happened to H. naledi? There are no answers to this question yet. But if the fossils really are just 300,000 to 200,000 years old, there is at least one possible scenario. Our species evolved in Africa at around that time. If those early H. sapiens reached southern Africa shortly afterwards, they might have contributed to the extinction of H. naledi.
Again, there is precedent for this. The fossil record elsewhere in the world shows that H. sapiens left Africa and gradually spread across Eurasia. As it did so, H. sapiens arrived in areas already populated by ancient humans – the Neanderthals and the Indonesian ‘hobbits’. Within a few thousand years of H. sapiens arriving in those areas, the indigenous species of ancient humans disappeared, apparently outcompeted by H. sapiens.
H. naledi might have the dubious honour of being the earliest ancient human species to have been driven to extinction by the spread of our species.
Hunting hominins
Lee Berger is the palaeoanthropologist behind the recent discoveries of two new species of human ancestor. The first was Australopithecus sediba, and it was the sort of once-in-a-lifetime find that most people in his line of work only dream of. If Berger had taken the conventional approach, he might have built the rest of his career on analysing it.
But following convention was not what Berger, an American who has made South Africa his home, had in mind. He was convinced that even greater discoveries were waiting, particularly in the ancient caves that riddle the limestone-rich countryside. He enlisted local help to search them, and in 2013 they struck it lucky: two chambers deep inside the Rising Star cave system contained hundreds of bones from another unknown species, which his team dubbed Homo naledi. This time, the story generated huge publicity.
So far, his team has found the remains of at least 18 H. naledi skeletons, of all ages. It’s a huge hoard, particularly because many hominin species exist only as a handful of bones. ‘There was a real perception that these fossils are rare,’ he says – and those who found them became reluctant to share access to such precious objects – ‘but they are not as rare as we once thought. We were looking in the wrong places.’ Asked about the scientific legacy he might leave, Berger’s answer picks up on this idea. ‘In 50 years, this might be looked on as the moment when we grew into an evidence-based science,’ he says.