5 Who were the ancestors of Homo sapiens?
Large brain size is a defining feature of Homo sapiens, which means that evolution of increased brain size in Homo is crucial evidence. Indeed, an increase in both the size and the complexity of the brain is a defining feature of primate evolution as a whole. It's possible to estimate brain sizes from fossil skulls or parts of skulls, e.g. by filling what there is of the skull with sand and then measuring the volume of the sand. Use of computer technology fills in 'gaps' in a fossil skull and provides a more complete estimate. Figure 6a is a plot of estimated brain volume for skulls of various species of australopithecine and named species of Homo against the dates for the skulls. Each red dot represents an estimate from one skull; the dots form a scatter plot, though there's no certainty that there is a straight-line relationship between the two variables.
Question: Study the data in Figure 6a. Read off the brain volume for Australopithecus, and describe the trend in brain size (volume) for the Homo species. (You may find it helpful to look back at Figure 3 to identify the different species of Homo.) Is there evidence of a sudden change in brain size?
The mean brain size for Australopithecus species was about 400-450 cm3. The oldest known Homo species, H. habilis, had a brain size of about 600-700 cm3; data for H. erectus are scattered, with values ranging from 750-1200 cm3. Mean brain size for Homo sapiens is about 1300 cm3, but the range is wide, from 1200-1700 cm3. Values overlap between succeeding species. There is no evidence of a sudden change in brain size - the rate of increase in brain size over this time period appears relatively constant. Overall, however, the increase over about three million years - from about 400 cm3 in Australopithecus, to 1300 cm3 in Homo sapiens - is a more than threefold increase.
Figure 6b shows the estimates of body mass of early Homo species. The mean value for Homo sapiens is not hugely different, demonstrating that the increase in brain size is disproportionate, not merely a reflection of increased body mass.
In the TV programme, DA describes one view of how natural selection promoted evolution of bipedalism. He suggests that early humanoids (hominines) discovered the rich food supplies available in shallow waters - molluscs and crabs. The idea that evolution of bipedalism links to the advantages of wading in water for obtaining a rich food supply ties in with the 'aquatic ape hypothesis', a contentious explanation for the evolution of humans. Supporters of this hypothesis quote our very sparse body hair [p. 297], the oily protective secretions of our skin and the layer of fat under our skin [p. 298] as supporting evidence. The subcutaneous fat is said to have provided insulation against low water temperatures for our ancestors. I find the arguments against the aquatic ape hypothesis more convincing; they include the fact that the distribution of human fat is virtually the same as that in furred terrestrial primates. Lack of fur in humans is more likely to derive from the advantages of being able to cool the body in a hot African climate, rather than from an aquatic lifestyle. (After migration out of warm regions of Africa, Homo used animal skins and furs for insulation against the cold.) There is no fossil evidence supporting an aquatic lifestyle for any of the species usually placed on the evolutionary tree for Homo. Needless to say, the arguments against the aquatic ape hypothesis are not accepted by its proponents.
However, the idea that early Homo found molluscs to be a good source of food is a sound one. DA links a diet of energy-rich shellfish to the huge increase in brain size in Homo [p. 298], though fish and a whole range of aquatic animals would also have provided much the same benefit. Let's look at the association between brain size and diet in Homo more closely.
The oldest fossils of Homo species, about 2.4 million years old, were found in Africa (see Figure 3). Rather than describe all such finds here, I'll focus on just one ancient species, Homo erectus. Fossil evidence suggests that H. erectus evolved about two million years ago in Africa and spread rapidly, migrating to Asia, reaching Georgia and East Asia. 'Peking Man' refers to H. erectus fossils found in China, at a site close to Beijing. Figure 7, below, compares the skulls of Homo sapiens and Homo erectus.
Question: Write a bulleted list summarising the differences between the skulls of Homo sapiens and Homo erectus.
This list highlights just the most striking differences between the two skulls:
Huge brow ridges are prominent in the Homo erectus skull, but not in the Homo sapiens skull.
Homo sapiens has a high vertical forehead, whereas the Homo erectus skull has a low flat forehead.
The teeth are larger in Homo erectus than in Homo sapiens.
There is a distinct bulge (formed from the occipital bone) at the back of the Homo erectus skull, which is not apparent in the Homo sapiens skull.
The face of Homo erectus is more massive and forward-projecting than that of Homo sapiens.
The mandible (lower jaw) is larger and longer in Homo erectus than it is in Homo sapiens.
The differences between the two skulls are so clear that you may now be wondering why Homo erectus is classified as Homo. Increased brain size is a major defining feature of Homo. As you've seen, available Homo erectus skulls have a brain size of about 750-1200 cm3, a considerable jump in brain size compared to 450 cm3 for Australopithecus afarensis (Figure 6). How can we link the increased brain size of Homo erectus to diet?
Brain tissue has a continuous high rate of energy consumption, so the larger the brain relative to body size, the greater the energy demand. LoM states that the human brain accounts for about 20% of the body's total energy expenditure [p. 298]. It is logical then to suggest that increasing brain size would have increased pressure on Homo erectus to find high-energy food. Fossil and archaeological evidence indicate that Homo erectus increased consumption of bone marrow and meat. Meat is a high-energy food that contains little indigestible material. Bone marrow, especially that of ungulates, is mainly fat, and so even richer in energy.
Drawing on your notes from the TV sequences showing how orangutans obtain fruit (Activity 2), how chimpanzees obtain fruit, leaves and meat (Activity 3), and how human hunters obtain meat (Activity 4), explain, in about 100 words, how techniques used for obtaining meat differ from those used for obtaining fruits and nuts.
Fruits and nuts are abundant seasonally and locally and require picking. Orangutans and chimpanzees have to travel to fruit-bearing trees at appropriate times. Access to the flesh of some fruits and the kernels of nuts may require removal of a hard husk. In contrast, the meat of wild animals is a food resource that has to be chased and caught (e.g. chimpanzee hunting colobus). Furthermore, prey animals are usually widely distributed, and mobile; a predator may have to travel many kilometres before finding prey, and needs to walk or run even further and much faster in order to catch it (San people).
Biologists describe meat as a high-quality food that is distributed in small patches, in contrast to leaves, which are an abundant low-quality food. Fruit is a high-quality food, distributed in occasional large patches.
As DA points out, early humanoids (hominines), did not have dagger-like canine teeth or powerful jaws, and could not run fast enough to catch an antelope [p. 298]. Modern-day hunter-gatherers cooperate with each other when hunting large animals and use weapons to kill the animals from a distance. Homo erectus is likely to have used similar strategies. We can imagine a group of H. erectus in which males cooperated both in hunting, and in defending their territory. H. erectus probably also scavenged the kills of carnivores such as lions or hyenas.
Sites where associations of fossil animal bones and stone tools have been found are interpreted by many archaeologists as areas used by groups of ancient hunter-gatherers. Site 50 at Koobi Fora in Kenya, dated at about 1.5 million years old, is one such example. Fossil remains of H. erectus have been found there, and also many tools made of local stones and many fossilised animal bones. Several stone flakes show signs of wood whittling and a few of them appear to have been blackened by fire. Evidence of fireplaces, basin-shaped reddish patches, was found at the site. This suggests that these groups of hominines, H. erectus, were processing and eating meat and bone marrow, spending at least some time at the site. In some bones, carnivore tooth marks are overlapped by cut marks made by stone tools, suggesting that hominines scavenged from carnivore kills. Other bones have tooth marks overlapping cut marks, indicating that hominines used the bones first. Therefore, the evidence suggests that both hunting and scavenging were important.
Although we are unsure about precisely how Homo erectus used supposed base camps, a picture is emerging of a complex social structure. Catching prey requires stamina and strength, together with skills such as designing and making stone tools. Increased brain size and complexity links to social skills, including planning, cooperation and communication between individuals. Another factor linked to increased brain size includes the long time taken for post-natal brain development in offspring. Infant helplessness means that females have to devote much time to caring for babies. Nursing females require support from a male partner and from the group as a whole. Prolonged childhood after infancy provides the time for infants to learn from their mothers and from other members of the group. When we put all these factors together, it becomes apparent that we are now looking at natural selection operating within a complex social group. We know that the basis for natural selection is reproductive success. Genes possessed by those individuals that produce the most offspring become prominent in a population, and also within a social group.
Question: Speculate on the features of an individual male Homo erectus that would increase his chances of reproductive success within a complex social group of hunter-gatherers.
Presumably those males who are the most skilled in obtaining food would leave the most offspring, because of their physical prowess, their social standing within the group, and also their ability to provide food for females with infants. The difficulties of hunting prey animals mean that individuals with both physical strength and social skills enabling them to make alliances, plan and cooperate with others would be the most successful. Physically weaker individuals who can build up alliances within the group may triumph over a stronger challenger, with the help of allies. Furthermore, for Homo - a meat eater - the ability to communicate with other members of the group would improve cooperation during hunting.
Forming alliances within a social group, and thereby gaining dominance, itself opens up opportunities for mating. Social interaction is important in primates for successful mating and production of offspring. For Homo, social interaction reaches levels of complexity not seen in other primate species. The ability to plan, predict consequences, cooperate and compete with others links to increased brain size and complexity. The effects of natural selection in a social environment would increase brain complexity still further. Such a process has been termed an 'evolutionary ratchet', which acts like a self-winding watch, increasing brain complexity and intelligence of the species.