This chapter provides a broad overview of primate diversity and evolution, as well as a relatively detailed accounting of early human evolution. It presents a summary of the major anatomical and behavioral differences between apes and humans and the adaptations involved. Humans diverged from the apes between 6 million and 8 million years ago and we have a reasonably good fossil record by 4 million years ago. We can readily track the major adaptive shifts in human evolution, as we became more bipedal and big brained, depending on stone tools by at least 2.5 million years ago. This chapter emphasizes the similarities between apes and humans, and the fact that, evolutionarily, we are apes. Importantly, although there are clear trends through time in hominins, human evolution is not unilineal, and there were as many as seven species and three genera of human existing across Africa at around 2 million years ago, and 27 human species are now known to have existed over the last 7 million years. We were an adaptively diverse group, and the modern idea of a single species of human is a result of a series of extinctions across the hominin order starting around 1 million years ago.

Just over 50,000 years ago – a blink in evolutionary time – there were at least six, possibly seven, human species alive: sapiens, neanderthal, denisova, floresiensis, luzonensis, remnants of erectus and, perhaps, the last of Homo naledi. This is similar to contemporary primates, with chimpanzees, gorillas, baboons and so forth all living side by side. It underlines the important fact that sapiens is one hominin species among many and that it is firmly part of nature and does not stand apart from it. However, we are used to occupying a unique solo position as the only humans, though such a situation accounts for less than 1% of the time since we separated from our last common ancestor with chimpanzees. This has led to the conceit and falsehood that evolution is a linear trail, marked through ever greater brain growth, leading to us. It also adds to the challenge when looking at the fossil record to determine whether some ancient species were indeed our ancestors or just side chains in a rich hominin tree. A further difficulty is that taxonomic descriptions imply clean leaps from species to species, such as between Homo habilis and Homo erectus. However, each child is a close genetic variant of its parents, and there are no hard borders as changes accrue across millennia. Late Australopithecus looks closer to the genus Homo than to early Australopithecines. Determining ‘when is a species a species’ is difficult (Barraclough, ). Adding to the uncertainty, interbreeding between different human species appears to be the rule rather than the exception, with there being complex flows of genetic material. It is a jigsaw for which we do not know how many pieces there are, nor how they might interlink. Further, the pieces are often found in fragments of single bones, and they can be so rare that many finds remain known by their ‘site name’. The last 10 years have seen enormous leaps in palaeoanthropology, from the discovery of previously unknown (and entirely unexpected) human species, to advances in molecular biology that have allowed us to sequence the Neanderthal genome and better estimate temporal links between fossils. Genetic data tell us that there are several other hominin species as yet undiscovered, whose ghostly footprints are currently seen only through a unique genetic imprint across some human populations.

Chapter 3 discusses bipedalism and stone toolmaking, considered the two first major milestones in the so-called "hominization process." The hominin record is succinctly explained with an overview of the contexts in which the first bipedal ancestors developed in Africa with a discussion of how and why first technologies might have emerged.

The next major entrant into the field of postwar paleoanthropology was Louis Leakey, a flamboyant character who had been scouring his East African homeland for hominid fossils since the 1930s. One major site of interest, in what is now Tanzania, was a large erosional gulley called Olduvai Gorge. And in 1959, in the Gorge’s oldest exposed sediments (known as Bed I), Louis’s archaeologist wife, Mary, discovered the cranium of a hyper-robust australopith that had tiny incisors and canines, and huge, flat cheek teeth (Figure 4.1, right). In the same year, they named this amazing specimen Zinjanthropus boisei (it is now regarded as a Paranthropus). The Leakeys had long known that Bed I contained very simple “Oldowan” tools consisting of rock “cores” from which sharp stone flakes had been bashed using hammerstones. The Oldowan material culture (the tools are often referred to as “Mode 1”) clearly preceded anything then known from Europe; “Mode 2” handaxes comparable to those at the bottom of Mortillet’s European sequence only appeared higher in the Olduvai geological section (though the Oldowan is now known in Europe, too).

There is little, if anything, in the hominin fossil record that can be said to closely anticipate the very derived anatomy of our species Homo sapiens. Compared to other members of the genus Homo, modern humans are exceptionally slenderly built, with narrow hips and barrel-shaped rib cages that taper both at the top and the bottom. Our skulls are high, short, and rounded (Figure 8.1); and instead of projecting, our very small faces are retracted beneath the front of our braincases. Our brow ridges vary from modestly protrusive to barely detectable, but they are invariably bipartite, with central and lateral surfaces separated by an oblique furrow. We are also alone among the hominins in having a true chin at the front of the lower jaw. This takes the form of an inverted “T,” with a vertical ridge in the midline of the jaw atop a horizontal bar running between a pronounced tubercle on each side.

The morphological medley of hominin crania known in Africa following the one-million-year mark indicates that the subfamily’s tendency to diversify continued unabated. It is frankly unclear exactly what was happening earlier in this period, but by about 600 kyr ago one hominin species had come to dominate the scene: Homo heidelbergensis. This was the world’s first-documented cosmopolitan hominid, with representatives from France, Italy, and Greece (Figure 7.1, left) in Europe; from South Africa, Zambia (Figure 7.1, center), and Ethiopia in Africa; and from China in Asia (Figure 7.1, right), apparently including the recently ballyhooed “Dragon Man” cranium from Harbin that was dubbed Homo longi. Dating of many of the known specimens is poor, but plausible dates as early as 600 kyr have been claimed for H. heidelbergensis in both Europe and Africa; and, the Dragon Man (>146 kyr) excepted, no proposed date for the species is more recent than about 200 kyr. Most of the fossils that represent H. heidelbergensis are cranial, and present us with a picture of a heavy-boned form with a modestly sized dentition and a reasonably large brain of between about 1,166 and 1,325 ml. Its face is massive, surmounted by very high brow ridges that show a characteristic lateral “twist.” Nothing like a complete skeleton of H. heidelbergensis is known, but the postcranial bones we do have are witness to a robust build, with a moderately wide pelvis and heavily built limbs of basically modern proportions. Allowing for flyaway hair and almost certainly some clothing in northern climes, on the landscape you would likely have had to approach to within a dozen yards of one of these hominids before clearly noticing that he or she looked rather different from you.

When Charles Darwin published On the Origin of Species in 1859, only a tiny handful of human fossils – the material evidence of the ancient human history that his book implied must exist – had been discovered. Some of them had not even yet been properly recognized, although the most significant of them – the partial skeleton from the Neander Valley in Germany that had recently been found alongside the remains of extinct Ice Age animals – was on the brink of becoming the world’s most famous fossil. Discovered accidentally in 1856 by lime miners, and only preserved by great good chance, the Neanderthal skeleton – principally a large skullcap (Figure 3.1, right) and some very robust limb bones – rapidly became the subject of vigorous debate in Germany between those who thought it had belonged to a member of an ancient barbarous tribe, and those who thought it simply the remains of a pathological modern human. In England it caught the attention of the comparative anatomist Thomas Henry Huxley, an expert on dinosaurs who had been Darwin’s most vociferous supporter following the publication of the Origin, but who had also chided him for rejecting the notion that Nature at least occasionally “makes jumps” (i.e., speciates).

Seven million years ago the continent of Africa, actively bulging upward along the north–south line of the volcanically active Great Rift Valley, was also experiencing climatic drying and increased seasonality of rainfall due to a general oceanic cooling. Particularly to the east of the Rift, the formerly ubiquitous forests were giving way to woodlands and bushlands, and even to some early grasslands, stressing the populations of large-bodied, tailless, and mainly fruit-eating apes that the Miocene forests of both Africa and Eurasia had harbored in profusion. But the stress of change also brought with it opportunity, in the form of the very different range of potential food resources offered by the expansion of more open environments. And while modern apes living partially in open environments tend to seek essentially the same resources there as those they exploit when living in closed forest, it appears that some archaic ape lineages were prepared to be a little more flexible, and to explore the new opportunities the expanding mosaic of environments had to offer.

Within the lifetimes of some of today’s older paleontologists, the armamentarium available to those who studied fossils was pretty limited, consisting mainly of hands, brain, and a rock hammer. A fossil, by the way, is any evidence of past life: An ancient footprint or worm burrow is technically a fossil, although as far as mammals like us are concerned the vast majority of fossils are the mineralized remains of bones and teeth. These are the hardest tissues of the body, and thus have the best chance of being preserved in the rock record. For reference, Figure 2.1 shows a human skeleton with the major bones identified.

The message from the population movements briefly recounted in the previous chapter is that Homo sapiens has been an itinerant species from the outset. Even in a world with intensively patrolled political borders there is no reason to believe that this propensity will be contained any time soon: It is an ongoing phenomenon that will have to be managed, hopefully as humanely as possible. More generally, the message deriving from the story told in this book is that we modern human beings have an astonishingly recent origin, and a sudden one. In evolutionary terms, we acquired our extraordinary symbolic reasoning capacities virtually overnight, and we did so exaptively (i.e., not in the context familiar today) rather than adaptively (within that symbolic context). What this most importantly tells us is that we cannot have been fine-tuned by natural selection over the eons to be the kind of creature we are today; there was simply not enough time. Which in turn suggests that, within the limits of circumstance, Nature has (albeit unintentionally) given human beings almost unlimited freedom to become the kinds of creatures they individually choose to be.

The notion that our planet and its inhabitants have not remained exactly as the Creator was supposed to have made them was in the air long before 1859, when the English natural historian Charles Darwin collected and published his evolutionary ideas in his great work On the Origin of Species by Means of Natural Selection. By that time geologists had long known that the 6,000 years allowed by the Bible since the Creation was vastly inadequate for the sculpting of the current landscape by any natural mechanism; and the biologists who were just beginning to study the history of life via the fossil record were not far behind them. Around the turn of the nineteenth century, the French zoologist Jean-Baptiste Lamarck began to argue that fossil molluscan lineages from the Paris Basin had undergone structural change over time, and that the species concerned were consequently not fixed. Importantly, he implicated adaptation to the environment as the cause of change, although the means he suggested – subsequently infamous as “the inheritance of acquired characteristics” – brought later opprobrium.

Once Louis Leakey and his colleagues had described Homo habilis in 1964 the search for his Holy Grail of the “earliest Homo” was on, and the field was wide open for other entrants that had very little in common with Homo sapiens, the species by which our genus is defined. Genera are hopefully monophyletic collections of species descended from the same common ancestor; but there are no formal rules governing how inclusive they can be, so you could in theory create a monophyletic genus by including every primate on the planet. By unwritten convention, however, zoological genera are in effect the largest readily (I almost wrote “intuitively”) recognizable unit, species being basically variations on the theme established by the genus. All species of the genus Felis are identifiably cats, and all Rattus are visibly rats. By this rule-of-thumb, as Bernard Wood and Mark Collard pointed out over two decades ago, the genus Homo should be confined to species that have significant resemblances to Homo sapiens. Just being related to the Homo clade, at some remove, is simply not enough.