by Holly M. Dunsworth
Humans would not have evolved if the ancestors of the African great apes had not. The ape fossil record begins 23 million years ago with the earliest putative apes, including Morotopithecus and Proconsul (Figure 1), from sites in East Africa, followed by many others throughout Africa, Europe and Asia. Although this record is fairly rich, it has done no better than DNA-based estimates at helping researchers to determine how living apes are related. Genetic studies estimate that gorillas split off from other apes about 9 million to 8 million years ago, and that the ancestors of bonobos and chimpanzees began evolving separately from the ancestors of humans 7 million to 6 million years ago.
Comparative anatomy, physiology, behaviour and genetics provide enough evidence for us to understand that humans are more closely related to chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) than to any other species, and vice versa. But the fossil record of hominins (species more closely related to humans than to chimps) preserves snapshots of the how the evolutionary path of our lineage differs from theirs. Unfortunately, the fossil record of chimpanzee and bonobo evolution is small enough to fit into a coat pocket, but the fossil evidence for human evolution is far greater: there are hundreds of specimens, including many nearly complete skeletons and many well-preserved skulls. Although the hominin fossil record is dominated by durable teeth — which reveal diet, age of death, pace of growth and much more — here we will focus, briefly, on the tales of two other significant human traits that are well documented in the hominin lineage: our big brains and our bipedal bodies.
Of course, humans are not the only animals to have extremely large brains for their body sizes (to be highly encephalized). Witness the octopus and the squid — members of the cephalopod class — and, among mammals, the toothed whales, or odontocetes. The African great apes also have large brains, but humans, as the sole surviving hominin, are considered to be the most encephalized. Nor are humans the only animals to walk habitually on two legs. Birds and many of their extinct dinosaur relatives are just some of the many bipeds that have roamed, and continue to roam, Earth. But although many primates, especially the African great apes, frequently walk on their hind limbs — particularly when carrying objects, while moving about the trees and during bouts of threatening or playing — humans are the only ones to be dedicated to this mode of locomotion.
The first five million years or so of the hominin fossil record (from about 7 million to 2 million years ago) are dominated by the gradual appearance of bipedal characteristics in the skeleton. It was not until the last 2 million years — by which point most of the skeleton, apart from the cranium (or top part of the skull), resembled that of a modern human — that encephalization took off.
Compared with other apes — for example, gorillas (Figure 2), which climb and hang in trees and walk on all fours using their manual knuckles — the human skeleton’s anatomy reflects adaptations for upright walking and running. The human pelvis is modified so that the ilia (the blades) are bowl-shaped and curved around to the sides of the body, rearranging the muscles for balance during the single-support phase (i.e. when only one foot is on the ground) that dominates the time we spend walking. The spine is curved at the lumbar (lower back) and cervical (neck) regions, balancing our skeletons. Human legs are longer than our arms and long for our overall size compared to apes, helping to make us better travellers. Our hip joints are large and sturdy, because only two limbs bear our weight. Our knees are also large and reveal the angle of our femur (thigh bone), bringing the knee and the foot directly under our centre of gravity with every step. Our ankles and heels are rigid bony blocks, and the arches of our feet help to store and release energy with each stride. Our hallux (big toe) is not able to grasp like the thumb-like toe of many apes, but instead lines up with the other digits (all short toes) and plays a role in forcefully pushing off from the ground (‘toe-off’) at the end of each step during walking and running.
The absolute best evidence for bipedal behaviour in the fossil record comes from footprints; they are direct impressions of that behaviour, requiring absolutely no inference from the shape of fossilized skeletons. And in Laetoli, Tanzania, there are wonderfully preserved 3.6-million-year-old tracks left by at least two bipedal hominins. They are not exactly like the prints that humans make today, but they lack an ape-like, divergent big toe and are not accompanied by any hand or knuckle prints.
At the time the tracks were laid down there is dental and bony fossil evidence in East Africa for Australopithecus afarensis. This is the species of the famous partial skeleton known as Lucy, discovered in the 1970s. Because A. afarensis skeletal morphology indicates that it walked upright, the Laetoli trackways are credited to the species. However, just whether A. afarensis walked upright all the time or only some of the time, and how much its bipedalism resembled modern human bipedalism, is still debated because A. afarensis did not have all the features that we associate with bipedalism in ourselves. This also goes for related australopiths discovered in South Africa, Australopithecus africanus and Australopithicus sediba. The australopith pelvis is not as bowl-shaped as ours; the legs are short and the arms relatively long; the toes are long and slightly curved; and the configuration of the tarsals, or foot bones, causes debate over whether the foot had an arch and whether australopiths tended to walk ‘pigeon-toed’. Making interpretation more difficult are new finds such as a foot from the site of Burtele in Ethiopia, which is near to and from around the same time as sites that produce A. afarensis fossils. The Burtele foot has some anatomy that suggests bipedalism, but also has an ape-like divergent hallux. It’s too much variation to include in a single species and, because of the hallux, cannot possibly belong to the hominins that left footprints at Laetoli. Despite these intriguing problems, it is clear that bipedalism, in whatever form it came, had hit its stride during Australopithecus times.
For many palaeoanthropologists, the presence of bipedalism is the standard way to identify a hominin, meaning to decide that a fossil is a member of the human family tree, not another ape’s. This is the main reason that australopiths are labelled as hominins. But australopith species are known to have lived only from a little over 4 million years ago to roughly 2 million years ago, which does not go far enough back to match DNA-based estimates of when hominins diverged from chimps and bonobos, around 7 million to 6 million years ago. There are fossils older than the australopiths that look tantalizingly like hominins, but not completely. They belong to three genera: Sahelanthropus (from about 7 million years ago in Chad); Orrorin (from about 6 million years ago in Kenya); and Ardipithecus (from between 5.8 million and 4.4 million years ago in Ethiopia). Tooth shape and indications that they walked on two legs mean that all three of these genera have been placed at the base of the hominin tree by some researchers. However, other researchers disagree, in large part because of debate about how these animals moved. Much more is known for Ardipithecus than the other two genera. As predicted for an early hominin, its skeleton has so many primitive and/or non-human-like features that it is not completely clear whether it was bipedal, and also whether it was an ancestor to australopiths (although its teeth suggest that it was).
For the foreseeable future, there will be debate about these early hominins and their behaviour: whether or not they walked on two feet regularly, they were doing so using a non-modern skeleton, so it is difficult to tell exactly how their movement worked. Bipedalism does not require a modern human skeleton, as shown by the Laetoli prints. However, the only way that researchers can work out how hominin fossils moved is to look at the observed anatomy and behaviour of the one surviving bipedal hominin species: modern humans. The traits that we associate with bipedalism in our own muscles and skeletons appeared slowly over the first 5 million years of hominin evolution, so those 5 million years are best described as showing a slow shift to habitual bipedalism.
There seems to have been an ecological shift to accompany the change in locomotion. Evidence, particularly from the chemistry of tooth enamel, suggests that australopiths were starting to eat lots of grasses and related plants, whereas other apes eat mostly fruits, leaves and nuts. This shift in dietary ecology supports the idea that the australopiths or their ancestors had moved out of the trees to look for food on the ground, consistent with a modified take on the ‘savannah hypothesis’ in which, during the Pliocene epoch (5.3 million to 2.6 million years ago), hominins evolved under pressure to be able to find food in the relatively new grasslands of East and South Africa. Instead of terrestrial bipedalism originating with scavenging and hunting behaviours, as in the usual savannah hypothesis, perhaps it began with a mainly herbivorous phase.
Another traditional scenario, suggested by Charles Darwin, is that bipedalism arose to free the hands for making and using tools, carrying tools and food, and throwing objects while foraging or socializing. Unfortunately for this hypothesis, there are few tools preserved from 7 million to 2.5 million years ago. If we accept that Ardipithecus, Orrorin and Sahelanthropus are early hominins, then we must say that bipedalism originated in wooded environments because that is how their environments have been reconstructed. The first hominins could have lived in trees as much or even more than extant great apes do now, and evolved bipedal locomotion there.
Evolving encephalization (Pleistocene, Old World)
Since the late Pliocene — when the hominin locomotor anatomy began to be familiarly human — hominin brains have tripled in size. Given that it is impossible to re-run evolution to find out whether our extreme encephalization could have evolved if we had not first become bipedal, we are all but forced to assume that bipedalism was a prerequisite.
There are three main hypotheses to explain hominin encephalization. The first is a technological scenario. Non-human primates that make and use tools have the largest brains and the most complex behaviours. Once the forelimbs are no longer necessary for locomotion, as in hapitually bipedal hominins, they can be used for more complex technology, more regularly, which in turn selects for further encephalization. This hypothesis is supported by the emergence of the first encephalized hominins — Homo habilis, the earliest members of our own genus — roughly coinciding with the earliest fairly regular appearance of crude stone tools, starting around 2.5 million years ago. These tools have been dubbed the Oldowan tradition, after Olduvai Gorge in Tanzania, where they were first discovered.
The second scenario to explain encephalization is ecological. Again, primates with complex ecological behaviours tend to have large brains. Once hominin bodies committed to bipedalism, they were suited for scavenging and hunting animal prey. Predicted consequences of this shift are borne out in the fossil record for Homo erectus, a hominin with half or more of the modern-human brain size, which emerged about 1.8 million years ago. The skeleton of H. erectus approaches modern proportions and the hominin’s anatomy seems to be built for short bursts of speed and long-distance travel. The diet included high-quality animal protein and fat for feeding a larger brain. H. erectus had a body size similar to that of modern humans (with lots of diversity), and it is the first hominin found outside Africa. Almost as soon as it originated in Africa, H. erectus dispersed across the continent and into Europe, central Asia and southeast Asia.
Much of the evidence for the ecological scenario is rooted in the discovery in the 1980s of a well-preserved H. erectus skeleton in Nariokotome in Kenya (Figure 3). It is not clear how large a role meat played in our ancestors’ diets, because the record is biased toward preserving bones of devoured prey over remains of devoured vegetation. However, there is no denying that an ecological shift occurred in the early Pleistocene, with H. erectus developing a more diverse diet and habitat and becoming more skilled at hunting. That shift must have included new requirements of the brain. There are hints at sites in Africa that H. erectus was able to control fire during the early Pleistocene, but the first reliable evidence of fire use does not appear until 800,000 years ago at a H. erectus site in Israel. Even then, there is no preserved evidence for regular fire use until around 400,000 years ago, when H. erectus was largely gone and more modern hominins existed. Scavenging, hunting, control and use of fire for cooking, living in diverse habitats in diverse climates and increasingly complex stone-tool manufacture require a larger, more complex brain.
The third major hypothesis for encephalization is social. As hominins became skilled hunters and gatherers, they relied more and more on cooperative foraging behaviours, and being able to navigate social networks across time and space became increasingly adaptive behaviour. Once complex speech and language arrived, there would be new demands on the brain, not only for these behaviours, but also for the new cultural, cooperative environment that language created. Brain size, and especially the ratio of brain to body size, reached modern proportions by 500,000 years ago, with ‘archaic’ or early Homo sapiens, so social selective pressures would have contributed both to reaching modern brain sizes and to maintaining it through to the emergence of modern Homo sapiens – represented by skeletons dating to 195,000 years ago at Omo in Ethiopia. The same social conditions might have led to encephalization among Neanderthals, Homo neanderthalensis, which lived roughly 300,000 to 30,000 years ago in Europe, the Middle East, and Eurasia. Neanderthal encephalization was comparable to ours and maybe slightly greater. There is an ongoing argument among researchers as to whether Neanderthals were a separate species, called Homo neanderthalensis, or a subspecies of Homo sapiens.
The technological, ecological and social pressures, requirements or demands could have worked both together and at different times throughout the Pleistocene epoch to the present and over many hominin generations. These evolutionary pressures would contribute to the more or less sustained reproductive success of hominins with slightly larger brains. And the fossil and archaeological records suggest that technology would have had the earliest effect, followed by the shift in ecology, and then sociality. These are some of the most mainstream hypotheses for encephalization and they implicitly or explicitly depend on bipedalism evolving before encephalization.
From bipedal ape to encephalized bipedal ape
We assume that some population of australopiths gave rise to the first members of the human genus, Homo. The earliest known Homo is australopith-like in anatomy but has a few differences, mainly its ever-so-slightly larger cranial capacity (a good proxy for brain size during life). Some researchers have argued that australopiths have marked encephalization, but others think that only early Homo had more encephalization than living apes. This debate will continue until more fossils are found that have indicators of both brain and body size for comparison. Until then, most researchers are comfortable beginning the brain-size story with Homo. It probably helps that the earliest stone tools on record and the earliest evidence for animal carcasses processed with those tools are found during the early Pleistocene, in early Homo times.
It is unclear when the hominin ecological shift to being stone-tool-making, meat-eating apes began. If the behaviour was common by H. erectus times, it should have started earlier. There is tantalizing evidence to suggest just this: the first known stone tools are from Gona in Ethiopia at about 2.6 million years ago, when only australopiths are thought to have been present. At 2.5 million years ago, when there were still just australopiths present, there are bones marked with cuts made by stone tools at nearby Bouri in Ethiopia. A few years ago, another site in Dikika, Ethiopia, dated to 3.4 million years ago (during A. afarensis times), produced what appear to be bones marked by stone-tool cuts. If this evidence is interpreted correctly, it is consistent with the dawn of an ecological shift leading up to the more conspicuous evidence with Homo erectus.
Considering the evolution of these two major traits from an energetic standpoint, bipedalism may have been a prerequisite for encephalization. Bipedal locomotion appears to expend less energy than walking terrestrially as a great ape and that freed-up energy could have been reallocated to brain growth. And, of course, with greater technological, ecological and social intelligence aiding human foragers the resulting increase in food quality and quantity provided the energy for growing a large hominin brain. Pound for pound, brains are metabolically expensive so even something as seemingly straightforward as natural selection for a large intelligent brain must be a complex story.
Palaeoanthropologists continue to strive for better methods of understanding behaviour from bones and describing the anatomical, ecological and environmental contexts of the origins and evolution of bipedalism and encephalization. New fossil, archaeological and geological discoveries will be crucial for solving these puzzles in palaeoanthropology.
For introductory information about human evolution see http://humanorigins.si.edu/ & http://www.pbs.org/wgbh/evolution/humans/index.html
A selection of peer-reviewed articles:
Bennett, M. R., Falkingham, P., Morse, S. A., Bates, K. & Crompton, R. H. 2013. Preserving the impossible: Conservation of soft-sediment hominin footprint sites and strategies for three-dimensional digital data aapture. PLoS ONE 8, e60755. doi:10.1371/journal.pone.0060755
Dunsworth, H.M. (2010) Origin of the Genus Homo. Evolution: Education and Outreach 3, 353–366. doi:10.1007/s12052-010-0247-8
Dunsworth, H. M., Warrener, A., Deacon, T., Ellison, P. & Pontzer H. 2012. Metabolic hypothesis for human altriciality. Proceedings of the National Academy of Sciences USA 109, 15212–15216. doi:10.1073/pnas.1205282109
Ferraro, J. V. et al. .2013. Earliest archaeological evidence of persistent hominin carnivory. PLoS ONE 8, e62174. doi:10.1371/journal.pone.0062174
Harcourt-Smith, W. E. H. 2010. The first hominins and the origins of bipedalism. Evolution: Education and Outreach. 3, 322–332. doi:10.1007/s12052-010-0257-6
McNulty, K. 2010. Apes and tricksters: The evolution and diversification of humans’ closest relatives. Evolution: Education and Outreach. 3, 322–332. doi:10.1007/s12052-010-0251-z
Strait, D. 2010. The evolutionary history of the australopiths. Evolution: Education and Outreach. 3, 341–352. doi:10.1007/s12052-010-0249-6
Tattersall, I. 2010. The rise of modern humans. Evolution: Education and Outreach. 3, 399–402. doi:10.1007/s12052-010-0241-1