This article is the first in a two-part series.
In pursuit of knowledge, the evolution of humanity ranks with the origins of life and the universe. And yet, except when an exciting find hits the headlines, palaeoanthropology and its related fields have gained far less scientific support and funding – particularly for scientists and institutions based in the African countries where so many landmark discoveries have occurred.
One of the first was made a century ago in Taung, South Africa, by mineworkers who came across the cranium of a 2.8 million-year-old child with human-like teeth. Its fossilised anatomy offered evidence of early human upright walking – and 50 years later, in the Afar region of northern Ethiopia that would become a hotspot for ancient human discovery, this understanding took another leap backwards in time with the discovery of Lucy.
The part-skeleton of this small-bodied, relatively small-brained female captured the public’s imagination. Lucy the “paleo-rock star” took our major fossil evidence for bipedal walking, human-like creatures (collectively known as hominins) beyond 3 million years for the first time. The race to explain how humans became what we are now was well and truly on.
Since then, the picture has changed repeatedly and dramatically, shaped by waves of new fossil discovery, technology and scientific techniques – often accompanied by arguments about the veracity of claims made for each new piece of the puzzle.
Even the term “human” is arguable. Many scholars reserve it for modern humans like us, even though we have Neanderthal genes and they shared at least 90 per cent of our hominin history from its beginnings around 8 million years ago. The essence of hominin evolution ever since has been gradual change, with occasional rapid phases. The record of evolution in our own genus, Homo, is already full enough to show we cannot separate ourselves with hard lines.
Nonetheless, there is enough consensus to thread the story of human evolution all the way from early apes to modern humanity. Most of this story centres on Africa, of course, where countries such as Kenya, South Africa and Ethiopia are rightly proud of their heritage as “cradles of humankind” – providing many of their schoolchildren with a much fuller answer then those in the west to this deceptively simple question: how did we get here?
Early apes to ‘hominisation’ (around 35m to 8m years ago)
The story of human evolution usually starts at the point our distant ancestors began to separate from the apes, whose own ancestors are traceable from at least 35 million years ago and are well attested as fossils. Around 10 million years ago, the Miocene world was warm, moist and forested. Apes lived far and wide from Europe to China, though we have found them especially in Africa, where sediments of ancient volcanoes preserve their remains.
This world was soon to be disrupted by cooling temperatures and, in places, great aridity – best seen around the Mediterranean, where continental movements closed off the Straits of Gibraltar and the whole sea evaporated several times, leaving immense salt deposits under the floor of the modern sea. Widespread drying was reported from around 7 to 6 million years ago, leading to a stronger expression of seasons in much of the world, and changes in plant and animal communities.
The divergence from the apes of a lineage – the hominins – that eventually led to us had probably already begun 8 million years ago. But our knowledge of this date depends on molecular comparisons with other animals, rather than fossils.
DNA shows we are most closely related to chimpanzees and their sister species, the bonobo. Branching points can be estimated by comparisons with other well-dated events, such as the separation of South American monkeys from other primates about 35 million years ago.
A surprise from genetic science is that gorillas, the other African great apes, are less closely related to chimpanzees than chimps are to us. A chimpanzee, if it could speak, might tell us: “These gorillas may look like my big brothers, but actually I’m more closely related to you.” They seem so similar because they are both tropical forest apes with similar adaptations, which underlines just how much – and how rapidly – the earliest hominins had to evolve to survive in their drier environments.
Yet, there is still some debate about whether the chimpanzee is our best model for the starting point: the “last common ancestor”. Better to call it the “best living model” because the chimp has shown many adaptations of its own, especially in its limb proportions and locomotion, but also in its large shearing front teeth. But its social behaviour, communication and tool-making have all provided invaluable insights into the processes that we can call “hominisation”.
Earliest hominins (about 7m up to 4m years ago)
The earliest hominin fossil yet known is about 7 million years old and comes from the middle of Africa, near Lake Chad. This rare find from 2001 is Sahelanthropus tchadensis, represented by a cranium (nicknamed “Toumaï” by its finders), a femur and teeth – all probably from the same species.
Although these finds were limited, they were enough to show a bipedal creature probably still comfortable living in trees, who had teeth with hominin features. Many accompanying fossils of other species show this hominin lived in both woodland and grassland habitats.
Then, for over a million years, our record vanishes – other than for some fragmentary remains of Orrorin tugenensis, a different genus of hominin found in the Tugen Hills of Kenya and dating to about 6 million years ago.
Hominins appear again in plain sight with a new species dating back around 5.5 million years, Ardipithecus kadabba. The discovery of its partial jawbone and teeth in the Middle Awash region of northern Ethiopia in 1997 shed more light on what may have been the “stem ancestor” leading to all later hominins. Exceptionally thorough investigations have since revealed these creatures in full anatomical detail and in remarkable environmental context, showing Ardipithecus combined characters of both apes and later hominins.
A. kadabba’s finders emphasised that it was not chimpanzee-like in limb proportions, nor did it have their exaggerated shovel-like front teeth. It also overturned the old theory of hominins coming down from the trees into savannas, and thus being forced to become bipedal. Rather, Ardipithecus lived in thick woodland and supports the idea that bipedalism first arose as an adaptation to walking along tree boughs, perhaps while clasping the branches above.
The stem hominin idea may well be correct, but more recent finds suggest there were soon multiple hominin species. While Ardipithecus is known from only one modern country, Ethiopia, there are huge areas of Africa that could have supported similar sibling species but which, for geological reasons, have not given up these secrets as generously as sections of the Great Rift Valley.
It is also striking that Ardipithecus’ feet remained apelike, with a divergent big toe – a sign that climbing trees was still important. The other, later species of Ardipithecus (Ar. Ramidus) lived only half a million years before the famous footprints found in Laetoli, Tanzania in 1976 – trails of footprints that displayed fully human characteristics. Evolution would need to have been rapid indeed for those two creatures to be directly related.
Even so, Ardipithecus had features that are enormously valuable for showing the general state of hominins at this time. Its pelvis, the oldest known, was short and basin-like as in later hominins, although ape-like in its lower part. And its teeth had enamel that was thicker than in African apes but thinner than in modern humans, suggesting an omnivorous diet.
Australopithecines (about 4.3m to 1.4m years ago)
More than 4 million years ago, another group of hominins begins to appear on the scene: the genus Australopithecus, named after the “Taung child” whose skull was discovered 100 years ago by workmen in the South African limestone quarry.
While the name means “southern ape”, the australopithecines were certainly hominins. Fully bipedal, their teeth were arranged in a modern human pattern with their canines reduced – sometimes to an extraordinary degree – and they existed in great diversity.
As finds accumulate, at least ten species of this group are now known, indicating “adaptive radiation” – meaning that hominins had become highly successful and were by now adjusting to many different habitats and climates. While the australopithecines were confined to Africa, they extended widely from the south to the east and even towards the west near Lake Chad – close to the find of the older Sahelanthropus. This distribution underlines the argument for hominins having originated in Africa, as had been long suspected from the shared heritage with African apes.
The oldest Australopithecus is A. anamensis, found in northern Kenya and dating to more than 4 million years ago, closely followed by A. afarensis in Ethiopia – Lucy’s species – and A. prometheus in South Africa.
Then, in addition to species such as A. africanus and A. garhi, there is a further group who combined enormous chewing teeth and ape-sized brains – their massive jaws and skulls led to them being dubbed the “robusts”. Often officially termed Paranthropus rather than Australopithecus, they occurred as three separate species in southern and eastern Africa, appearing at least 3 million years ago and surviving until about 1.4 million years ago.
The Great Rift Valley, Kenya. Photo by Ninara (Flickr CC)
While microwear studies of their teeth suggest a mixed diet, the huge size of those teeth implies it was of low quality, with grasses and sedges providing the bulk. Indeed, the dominance of these creatures’ massive molars meant their front teeth shrank to the extent that their incisors and canines were consistently smaller than ours today.
Although the African Rift Valley running down the east side of the continent is often celebrated as the focus of hominin origins, the distribution of australopithecines is just wide enough to show the rift is not necessarily the cradle of humankind – although it is the region where most fossils have been found. South Africa’s dolomite caves are strong competitors in importance, while the discovery of A. bahrelghazali in Chad is far west of the rift.
Beginnings of Homo (from about 2.8m years ago)
It is certain that our own genus, Homo, emerged at some point from within the australopithecines. But exactly how and when is still difficult to ascertain, because cranial remains – skulls – are very scarce in the period between 3 and 2 million years ago.
This is a matter of chance; before and after, we have plenty of them. Large numbers of teeth prove that hominins were in eastern and southern Africa during this period, and rare finds of crania such as P. aethiopicus and A. garhi make the point that others could be found at any moment.
In later times, Homo is distinguished by its very large brain – about three times the size of a chimpanzee’s brain – but this was not so in the beginning. At the start, Homo would have been almost indistinguishable from australopithecines, with just some small anatomical details picking it out, especially the shape of its molar and premolar teeth. Fragmentary jaws and teeth from Ledi Geraru and Hadar in Ethiopia, then from Chemeron in Kenya, trace the early story of our direct ancestors from 2.8 to 2.4 million years ago.
As we approach 2 million years, Homo appears much more clearly in famous skull and other fossil discoveries from Olduvai Gorge (Tanzania) and East Turkana (Kenya), and latterly South Africa. As well as at least three species in Africa – Homo habilis, Homo rudolfensis and Homo erectus – Homo of similar age is suddenly found outside Africa, especially at Dmanisi in modern-day Georgia, where the finds are as old as those from Olduvai.
Together with first archaeological finds of stone tools and cutmarks on animal bones indicating butchery, these fossils combine to show us that Homo had become highly successful within a million years of its origins, and had spread out far across Asia as far as China. These first arrivals must have been a species of early Homo, but it is only at Dmanisi and Lantien in China that we have early fossil remains.
Technology was almost certainly part of the adaptation that allowed this great expansion. Tool-making is the most obvious part of early cultural behaviour, and it is preserved as hard evidence by the presence of stone tools.
First dates for stone tool-making have moved back in an exciting way. The 2 million-year barrier was broken around 1970, then the 3 million-year barrier just recently with discoveries of stone tools at Lomekwi and Nyayanga in Kenya. We do not know who made these tools, but it means stone artefacts emerged around the same time that early Homo appeared alongside the australopithecines. As “stone age visiting cards” – as the archaeologist Glynn Isaac labelled them – they are hugely useful for telling us where hominins went, and what they were doing.
A generation ago, it would have been axiomatic that the emergence of tools and Homo were linked, and that they signalled a major step forward – the development of early human culture. Now, there is a different perspective, largely owing to detailed studies of living animals.
To a striking degree, chimpanzees make a range of tools as well as use them, and so do the small capuchin monkeys of South America. Birds too are in the picture, especially the New Caledonian crow. Their tools may be simple – mainly made from plant materials – but they include stones used for hammering.
There are many indications that this animal behaviour is cultural, handed on as learned tradition. Granted that we, Homo sapiens, are the most cultural animal of all, there has to be a possibility that all hominins were toolmakers and users, given that all fossil hominins are more closely related to us than to the chimpanzee, which is itself a habitual toolmaker.
Having said that, we don’t know who made the earliest stone tools. We know that when Paranthropus and other australopithecines eventually disappeared, toolmaking continued – but this does not rule out earlier tool use by some of them.
Most early stone artefacts, from about 3 to 1.8 million, are placed in the “Oldowan tradition” – named after Olduvai Gorge where so many tools have been discovered, typically made from carefully selected lava or quartzite rocks. Both heavy core tools such as “choppers” and sharper stone flakes were used for a variety of tasks – certainly including animal butchery, and almost certainly in the preparation of plant foods and shaping of wooden tools (although these did not survive for our discovery until much later).
This toolkit literally gave early Homo species a cutting edge in the struggle for survival in varied environments, and may have been a key factor underlying their ability to expand their niche into new areas, including Jordan, north India and China well over 2 million years ago.
This article was originally published in The Conversation. It is republished here as a two-part series.
Main photo by MediaEcke on Unsplash.
