Human Origins: 7 million years and counting

The imagery of human evolution sometimes gives the impression of a thrusting, determined species, labouring to become a success regardless of what was happening in the world. But, in fact, humans and our cousins are just like other species: we are part of a wider ecosystem; and the course of our evolution has been affected by the places we have lived and the species with which we have coexisted. If the world had been different, we would be different; we might not even be here at all.

But, by the 1990s, Rick Potts from the Smithsonian Institute in Washington, DC had a new theory. He had concluded that the critical part of the human evolutionary story is that our lineage became extremely versatile, capable of living in all kinds of habitats. We are not masters of savannah life but master invaders. This led Potts to suggest that maybe it was environmental change itself – not a particular environment – that drove human evolution. A variable climate, he argued, would have placed a premium on being nimble and versatile.

In 1996 Potts published the idea in his book Humanity’s Descent, calling it ‘variability selection’. In 2015 a series of papers by Potts and others finally presented evidence. What had been missing before was a clear link between periods of highly variable climate and milestones in human evolution, so, over two decades, Potts and others gathered evidence of past climates at sites where early humans lived. This allowed them to pinpoint periods of highly variable and stable climate at five such sites in Africa, from between about 3.5 and 1 million years ago.

However, it is not clear that any of these major changes in humans were actually a result of natural selection. It is just as likely that what Potts attributes to natural selection might actually be other types of evolution, such as random genetic drift, which would mean that climate did not have an important role at all. These uncertainties notwithstanding, the correlations between human evolution and climate change keep piling up.

One such piece of evidence comes from East Africa’s Turkana Basin. Part of the Great Rift Valley straddling Kenya and Ethiopia (see Figure 9.2), Turkana has yielded many crucial hominin fossils. But was Turkana a recurring site for major events in human evolution, perhaps because it was a humid refuge for our ancestors during particularly dry periods, or was it simply a good environment for preserving fossils?

East Africa as a whole became drier between 3 and 2 million years ago – the period when our genus, Homo, first emerged. But the Turkana Basin began to dry out earlier, which means that it could have acted as a ‘species factory’. New species that evolved there were adapted for the drier environment that later became widespread. They would have been effectively ‘ahead of the trend’.

The drying coincides with many major events in human evolution, including the appearance in the fossil record of the first members of the Homo genus, along with Paranthropus, a group of hominins known for their robust skeletons and grinding teeth. Australopithecus disappeared at around the same time. The specific role of the climate shift in these events is unclear, but it would have changed what foods were available.

Archaeologist Geoff Bailey of the University of York in the UK and Geoffrey King of the Paris Institute of Earth Physics in France have spent decades amassing evidence for the theory. They argue that our ancestors evolved into modern humans while inhabiting tectonically active regions. Intelligent species would have thrived in these deformed landscapes, exploiting the topography to hunt, avoid predators and competitors, and build defensible homes. Eventually, they developed large brains, a prolonged childhood and the use of advanced tools and weapons. Less smart species would not have had the ability to use the uneven ground to their advantage.

The Earth’s surface is divided into plates, which move around over the millennia. Where they grind against each other, pressure builds up, and this can trigger earthquakes and volcanic eruptions. But Bailey and King are concerned with subtler effects. In active regions, the folding and faulting of the crust, combined with regular earthquakes and volcanic activity, create a disrupted landscape with many hills, valleys and cliffs, crisscrossed with solidified volcanic lava.

Bailey argues that these complex landscapes were perfect for early humans, who were not fast runners or particularly strong, but intelligent and adaptable. For instance, even though weapons such as spears had not been invented, early hunters could kill large animals by exploiting the irregularity of the landscape. What’s more, since humans evolved from tree-living primates, they would have found it easy to switch to clambering around hills and valleys. By contrast, they would have been at a disadvantage on flat, open plains like the African savannah, which is dominated by fast-running predators like lions and hyenas.

Tectonically active landscapes are also more likely to have reliable water sources, because earthquakes can trap water behind barriers of rock, forming lakes, and underground water can rise through faults to form springs. These water supplies would support plants and attract animals. Barriers like cliffs and ridges would have made life safer by allowing early humans to hide from predators and defend themselves against invaders.

If this theory is right, we should find that early humans were clustered in tectonically active regions. In a 2010 study Bailey and King superimposed the locations of human fossil sites throughout Africa with satellite images that show the roughness of the land, and found that they lined up neatly. In fact, 93 per cent of the fossil sites are in regions of high or medium surface roughness. For example, most of the classic human fossil sites, like Olduvai Gorge and Laetoli, are found along the East African Rift, where two continental plates are slowly coming apart. Bailey and King have found similar patterns of fossil sites in Arabia, which humans colonized later.

However, that left a big problem. Buried remains are more likely to be thrown up on to the surface if they are in an earthquake-prone region, so the results could be misleading. To get around this, Bailey and King, with Sally Reynolds of Bournemouth University, UK, extended their studies to South Africa, where human remains have been found in sites including Taung and Makapansgat. Rather than having been revealed by tectonic activity, the remains were found in caves. There are hundreds of such caves, but only some have remains and those are in regions that were tectonically active, they found.

Toba is a supervolcano on the Indonesian island of Sumatra. It has blown its top many times but one eruption, 74,000 years ago, was exceptional. Releasing 2,500 cubic kilometres of magma – nearly twice the volume of Mount Everest – the eruption was more than 5,000 times as large as the 1980 eruption of Mount St Helens in the USA, making it the largest eruption on Earth in the past 2 million years. The disaster is particularly significant since it occurred at a crucial period in human prehistory – when Neanderthals and other hominins roamed much of Asia and Europe, and around the time that our direct ancestors, Homo sapiens, were first leaving Africa to ultimately conquer the world.

Our ideas about this eruption have changed a great deal since 2000. Previous computer models had suggested that the event was truly cataclysmic – very nearly a doomsday for early humankind. With calculations based on the assumption that Toba belched out 100 times more aerosols than the 1991 eruption of Mount Pinatubo in the Philippines, and scaling the environmental effects accordingly, the models suggested that global temperatures dropped by about 10 °C following the blast. This supports the idea of a decade-long ‘volcanic winter’ and widespread catastrophe.

Indeed, the event may have drastically altered the path of evolution for our own species, Homo sapiens. Modern humans, who were still thought to be living in Africa, would have been reduced to just a few thousand breeding pairs scattered in dispersed refugia – creating a ‘genetic bottleneck’ in evolution. As the separate colonies developed independently of one another, they would have sown the seeds for the genetic differences between races once these separate groups eventually left Africa.

This theory has drawn criticism in recent years. Scholars such as Hans Graf, an atmospheric scientist at the University of Cambridge, now believe that the climate change following the explosion has been wildly overestimated. He and his colleagues have suggested a new estimate of global cooling of 2.5 °C lasting just a few years. According to this model, the effects were also highly regional. In places like India, the average temperatures may have fallen by only about 1 °C – not such a dramatic climate shift.

To build further evidence, Ditchfield analysed the ratio of different carbon isotopes – which are each absorbed at different rates by different plants – in ancient plant remains in the Jwalapuram region of southern India and the Middle Son river valley in central northern India, both of which are around 2,000 kilometres from Toba. He saw only a slight increase in the carbon-13 isotope after the Toba eruption, which suggests just a small increase in grassland environments at this time. In other words, woodlands were not obliterated by Toba.

Nevertheless, hominin species living at the time of the eruption would undoubtedly have faced tough conditions. The blanket of ash, for example, would have been quickly washed into the freshwater supplies: Ditchfield found deposits up to 3 metres deep on the valley floors where rivers would once have flowed. And there is no doubt that in the years immediately following the eruption the early humans would have had to adjust to colder temperatures, probably having to economize significantly as food resources dwindled. But just because life was difficult for humans after Toba does not mean that the situation was catastrophic.

Mike Petraglia at the Max Planck Institute for the Science of Human History in Jena, Germany, led a team to investigate a number of sites at Jwalapuram. One has been particularly fruitful. Labelled Jwalapuram 22, it was probably a hunter-gatherer camp. It has yielded more than 1,800 tools, including stone flakes, scrapers and points – the everyday tools for cutting and scraping – and the stone ‘cores’ left over following tool manufacture.

Surprisingly, hominin life appeared to continue in the same vein immediately after the eruption, with hundreds more stone tools in the layers immediately above the ash fall. The team uncovered a similar story 1,000 kilometres further north of Jwalapuram, in the Middle Son river valley. Again, that is not to say that the eruption was an easy ride for the hominins living in India. Jwalapuram and the Middle Son valley may have been special cases – refugia in which hominin populations sheltered when times got tough. Still, the findings present a challenge to the traditional view of Toba as a devastating catastrophe for hominins alive at the time.

The impact of inbreeding

Another key factor shaping our evolution may have been rather inauspicious. It seems that, for thousands of years, our ancestors lived in small and isolated populations, leaving them severely inbred. The inbreeding may have caused a host of health problems, and it is likely that small populations were a barrier to the development of complex technologies.

From the sequenced genomes of Neanderthals and Denisovans, David Reich of Harvard Medical School in Boston, Massachusetts found that both species were severely inbred, due to their small populations, and had an extraordinarily low level of genetic diversity. This is in line with previous evidence of small populations. It has been estimated that, in the distant past, human populations were probably only in the thousands or at best tens of thousands.

Fossils suggest that the inbreeding took its toll, according to studies by Erik Trinkaus of Washington University in St Louis, Missouri. Those he has studied have a range of deformities, many of which are rare in modern humans. He thinks such deformities were once much more common.

Such small populations may have affected the course of culture and technology. Larger populations retain more knowledge and find ways to improve technologies. This ‘cumulative culture’ is unique to humans, but it could emerge only in reasonably large populations. In small populations knowledge is easily lost, which explains why skills like bone-working show up and then vanish.

Tiny populations may have prevented Neanderthals and Denisovans from developing cumulative culture, placing limits on their cultural complexity. The same thing held our species back, until the population reached a critical density, unleashing the power of culture – at which point there was no stopping us.

One further influence on human evolution is our capacity for caring for other people’s offspring. As we have seen, humans are an unusually cooperative species, and this may be key to our success. A handful of researchers have argued that cooperation arose from the development of a single behaviour: sharing childcare. They claim that the care, nutrition and protection of youngsters by adults other than the mother bring about profound psychological changes in a species. In humanity, this may have paved the way for the enhanced cooperation and altruism that led to culture, language and technology.

There is no doubt that humans are extraordinarily social when compared with most of the animal kingdom. We are generally good at reading other people’s emotions and adapting our behaviour appropriately, we work well in teams on highly complex projects, and we sometimes even extend our kindness to perfect strangers. This capacity for cooperation is thought to have been essential for the development of culture and technology, making it one of the defining changes in our evolution. So where did it come from?

Chimps have a mean streak, but monkeys called marmosets are remarkably altruistic, much like humans. Carel van Schaik and Judith Burkart began to wonder what might explain these acts of altruism. One similarity seemed to stand out: humans and marmosets are ‘cooperative breeders’. Much more than most other primates, the adults of a marmoset group willingly protect and actively feed one another’s young, usually without any prompting. Chimps, by contrast, are independent breeders who will rarely help another’s family. They are not even particularly giving to their own infants.