Figure 4: The hominin family tree
What we do see depends mainly on what we look for … In the same field the farmer will notice the crop, the geologists the fossils, botanists the flowers, artists the coloring, sportsmen the cover for the game. Though we may all look at the same things, it does not all follow that we should see them.
John Lubbock
THOUGH DARWIN INITIALLY SEEMED WRONG in his Africa-first hypothesis, the first evidence that he might be right came from a German geologist, Hans Reck, shortly before the First World War. Not only was Reck the first European to behold the Olduvai Gorge in Africa’s Great Rift Valley, his team was the first to recognise a hominin fossil there.
As confirmation of Darwin’s theory and for palaeontology more generally, the Great Rift Valley is vital and famous in the study of human evolution because of the fossil riches preserved by its unusual geological properties. The term originally described a 3,700-mile trench running from Lebanon to Mozambique, a fascinating geological formation that emerged from the splitting of the earth’s crust. However, most researchers today understand ‘Great Rift Valley’ to refer to something smaller, to the part of East Africa where new tectonic plates are forming and literally beginning to tear the African continent apart. To find such a place anywhere on earth is to find a time-machine. Descending through the geological layers in the Rift Valley is like travelling back through history to prehistory, a journey of several million years. Even though the interpretations of finds in the valley are often complicated by mixing and corruption of fossil sites by tectonic upheaval, flooding, volcanic activity and so on, the Great Rift Valley has been and continues to be of inestimable importance for evolutionary theory.
When he was there in 1913 to study the earth’s geological history and to excavate fossils, the twenty-seven-year-old Reck recognised this. And his work paid off. Near the end of three months of hard work, in the formidable East African equatorial heat, a crouching skeleton was discovered in one of the oldest layers of the gorge. Reck recognised that the remains he had discovered were of a Pleistocene Homo sapiens who probably had drowned there some 150,000 years ago.
The year was an ominous one, of course. Soon the ‘War to End All Wars’ began and palaeoanthropological research was suspended in order to carry out the sinister work of mass killing. For that reason, not much else was to come from Olduvai until the arrival of Louis Leakey more than twenty years later.
Leakey was a controversial researcher who energised palaeoanthropology in much the same way that Chomsky did linguistics and Einstein physics (though Leakey did not lead as a theoretician). He shook up his field by grandiose claims that attracted publicity both to the field and to Leakey himself. Along the way he and his family discovered some very important fossils in East Africa. Louis also fostered research on primates in their natural habitat, recruiting and encouraging researchers such as Jane Goodall (chimpanzees), Dian Fossey (gorillas) and Birutė Galdikas (orang-utans) to undertake their own field research.
After both advancing the earlier research of Hans Reck and eventually working alongside Reck himself, Leakey and his team discovered artefacts such as Olduwan and Acheulean tools, a skull of Paranthropus boisei, then called Zinjanthropus, and Homo habilis, among many others. Leakey and the headline-making publicity he received attracted many scientists to palaeoanthropology. Whatever his shortcomings, he earned his place as one of the innovators and founders of the field of palaeoanthropology.
More importantly, the findings of Leakey and other palaeoanthropologists have provided incredible insights into the evolution of our species. We now know that the human skeleton evolved over the last 7 million years or so, from the first likely hominins. Some of the features that distinguish us from other species include bipedalism, encephalisation, reduction of sexual dimorphism, hidden oestrus, greater vision and reduced sense of smell, smaller gut, loss of body hair, evolution of sweat glands, parabolic U-shaped dental arcade, development of a chin, styloid process (a slender piece of bone just behind the ear) and a descended larynx. These traits have become important to the classification and understanding of the place of different fossils in the hominin line.
One of the adaptations of human skeleta to the world around them came as evolution provided a novel form of locomotion. Humans are the only primates that walk upright. Other primates favour crawling or tree-swinging to get about. But to walk habitually (unlike a chimp, an orang-utan, or a bear that can walk upright only occasionally and for brief periods), our skeleta needed to change from the basic primate model to support this upright posture. One example of its many changes is found in the hole at the bottom of our skulls, called the foramen magnum. This is the aperture through which our spines connect with our brains. When this is found at the back of the underside of the cranium, we know that the creature did not walk upright regularly, because it would have been extremely uncomfortable. The spine would emerge nearly parallel to the ground for a creature on all fours, but awkwardly incline the head if the creature walked upright.
Another important milestone, the human head and brain, was achieved by a long process of encephalisation, the gradual process by which our brain cases got larger. Hominin brain case volume increased from about 450cm3 for australopithecines to sapiens’ 1,250cm3. The heads of hominins show larger and larger brain cases until the appearance at Homo sapiens (neanderthalensis had even bigger brains than sapiens, averaging about 1,400cm3 for males). Sapien skulls are large, rounded and delicate compared to the smaller brain cases and thicker skulls of our hominin ancestors. Gone are the special ridges at the top of hominin skulls to anchor muscles for chewing, along with the heavy brow ridges that perhaps shaded our eyes from the sun. In their place came a larger brain. And our heads developed accordingly, to give room and horsepower for thinking.1
Male and female bodies also grew more similar in size – that is, our sexual dimorphism was reduced. Although human males are roughly 15 per cent larger than human females on average, this size difference is smaller than that of any other primate species. The reduction of sexual dimorphism in the primate line has social implications. When males and females become more similar in size, this correlates, among primates, with pair-bonding, or monogamy. Male primates spend more time helping females feed and raise children. This is particularly important for human primates, since our children require a longer time to mature.
In some Western, industrialised cultures as much as one-third of a person’s overall life expectancy is ‘childhood’ – the length of time required to reach autonomous adulthood. If males and females bond for life or simply in order to raise children, then the male will no longer need to battle other males for mating access. This reduces the pressure for males to have larger physical size, longer canines and other features for fighting. Battle is no longer necessary in order to pass our genes along to the next generation.
Along with bipedalism and reduced sexual dimorphism came a greater reliance on vision. Humans can see further than other primates, and most other creatures, which enables them to run faster towards a visible goal. Moreover, beginning with the arrival of Homo erectus humans acquired the capability of ‘persistence hunting’, running down game until it tires and the hunter kills it with a stone axe or club, or until it dies of exhaustion and overheating. Persistence hunting is seen even today in societies such as the Gê communities (Mebengokre, Kĩsedje, Xerente, Xokleng and others) in the savannah regions of the Brazilian Xingu river basin.*
Evolution is also the ultimate economiser. With humans’ greater dependence on vision came a loss of acuity and range in their sense of smell. If one portion of the brain gets bigger or better, another part very often grows smaller in the course of evolution. Here, the ability to smell degenerated as the vision region of human brains grew. Today the portion of the brain available to vision is roughly 20 per cent. (Fortunately, if someone is born blind, the vision region can be enlisted for other functions – evolution is often an efficient, no-waste process.)
Other changes to human physiology, not all with an immediately obvious intellectual benefit, might also have enhanced our species’ intelligence. In the course of evolution, the length of the gut of hominins shrank. Intestines and digestive processes required fewer and fewer overall calories, enabling Homo bodies to shift more of their energy resources to their growing, ever-hungry brain with its expanding cranium. But natural selection does not receive all the credit for this change. Cultural innovation also played a role.2 Homo erectus learned to control fire as long as one million years ago. As pre-erectus hominins learned to eat cooked food, the fats and proteins that they then ingested were much easier for their digestive system to break down. Whereas until this time, hominins, like other primates, needed larger guts in order to break down the large amounts of cellulose in their diet, as hominins learned to cook they were able to eat more meat and consequently able to consume more energy-rich food and to reduce significantly their dependence on uncooked plants that were much harder to digest. This fire-enabled dietary change facilitated natural selection’s ability to produce larger brains in hominins, because their digestive organs required less energy and less space in the human body, while at the same making it possible to consume far more calories far more quickly (assuming the availability of meat). Cooking also altered our faces. It rendered the massive jaw muscles of pre-Homo hominins redundant and made our faces less prognathous. This in turn reduced the burden on the cranium to offer supporting structures, such as the mid-sagittal crest of the australopithecines, that arguably impeded the growth of the human brain case.
There are critics of this change-through-fire hypothesis. They hypothesise that Homo erectus was a scavenger and a hunter, finding rich sources of meat from carrion and fresh kills of its own long before controlled fire. Whatever the reasons, this reduction in gut size represents the movement to modern human anatomy. When encountered in the fossil record it is therefore a clue that the species represented by the particular fossil could be further along the evolutionary line to Homo sapiens.
Other important physiological changes needed for us to become modern humans included our upright posture and its by-products. As humans stood erect and walked habitually upright, their bodies became more efficient at thermal regulation. Moreover, since an upright body’s surface areas are less exposed to direct sunlight than a quadruped’s, hair became less necessary for humans. As a side benefit of shedding body hair, it became easier for humans to cool their bodies. They also evolved sweat glands in conjunction with the hair loss, making thermal regulation much more efficient. In hot, dry climates, the absence of hair and the production of perspiration allowed humans to cool off more quickly than many other animals. And sexual selection may have further sped up hair loss if people preferred less hirsute mates. This was all important to sustaining the human metabolic rate, so crucial for our intensely calorie-consuming brains.
Another characteristic of modern humans is their parabolic dental arcade. The evolution of human dentition has many causes and effects and dentition is important to fossil classification. Homo species’ teeth shrank relative to their overall body size. Its canines in particular became smaller, which is important because this meant that Homo males no longer needed the larger teeth of other primates in order to fight for mating rights.
As the human dental arcade became more parabolic in shape, their faces came to possess more space for the articulation of different consonants and greater resonance for vowels, making a larger array of sounds available for human speech.
In order to summarise the output of human evolution in relation to other primates and to better understand the fossil record, a review of the primate phylogenetic tree on page 24 above is important.
Figure 4: The hominin family tree
Taking discoveries in order of the age of the fossils found, the first ‘node’ in the primate tree that links to humans (and chimps) is quite possibly Sahelanthropus tchadensis, literally Man of Chad of the Sahel area (‘sahel’ being cognate with Sahara, which gives name to what is today the largest desert on earth). Sahelanthropus is a potential direct link between humans and chimps but the more likely hypothesis is that it, like Orrorin tugenensis and Ardipithecus, was one of the first hominins as in the lower right portion of Figure 4. The repetition of the names above and below the split indicates the two major hypotheses regarding these hominin fossils. It lived some 7 million years ago. Though we have only parts of the cranium, mandibles and teeth of Sahelanthropus, it is important to the fossil record of the evolution of Homo sapiens because it represents several distinct but equally important possible clues to human evolution. It could even be the ‘node’ in the phylogenetic tree from which chimpanzees and humans split (Figure 4).
Before concluding the discussion of the rise of hominins, it would be useful to compare what is emerging about the cognitive and communicational abilities of other great apes in order to more effectively reflect on the nature of the different evolutionary paths – if any – to language.
Mammals are the most intelligent creatures of the animal kingdom. Primates are the most intelligent mammals and humans are the most intelligent primates. Therefore, humans are the most intelligent animals. This may not be saying much. After all, our intelligence is the reason we murder one another and fight wars. Our brains are a mixed blessing. Jellyfish get along quite nicely without brains.
Nevertheless, human language is possible only because of this greater intelligence. Humans are the only creatures known that use symbols and cooperate to communicate more effectively. And unlike other animals, when humans communicate they almost never say all that they think, leaving their hearer to infer meaning.
There are some prerequisites, or what are often called ‘platforms’, needed for language.3 Two of those are culture and ‘theory of mind’ – an awareness that all people share cognitive abilities. Culture is an important topic, but right now it is worth discussing the theory of mind, because this also helps gauge an area of cognition where humans are regularly claimed to have something that no other animal has – the ability to ‘read’ the minds of others. Although actually being able to see or hear the thoughts of others is science fiction, there is some truth to the idea that humans can guess what others are thinking and then use this knowledge as a key to communication.
To cognitive scientists such as Robert Lurz, mind-reading is ‘the ability to attribute mental states, such as beliefs, intentions and perceptual experiences, to others by the decidedly mundane and indirect means of observing their behaviours within environmental contexts’.4 An example of this might be to see a man with two bags of groceries standing outside the entrance to a house feeling around with his free hand in his pockets. A person with a knowledge of locks and keys and the custom of locking one’s house should be able to guess that the person is searching for his keys and that he plans to unlock and then enter the house. Even for this seemingly simple scenario, there is a huge amount of cultural knowledge being drawn from. Amazonians who lack locks and keys might not have a clue as to what the man has his hands in his pockets for. And yet all humans will most likely recognise that the man has an intent, a purpose or a goal; that his actions are not random. That’s because all humans have very similar brains, which is in essence what the theory of mind is. Folks with autistic spectrum disorder might not understand this, because there is reason to believe that some forms of this set of ailments are caused by a lack of this kind of awareness.
Language works only because people believe other people think enough like they do to understand what they want to tell them. When one says what one is thinking, they do so believing that their hearer will be able to understand, infer conclusions about and match our words to their own experiences. Therefore, the question that arises is whether humans alone in the animal kingdom have this ability. If other creatures possess it, what does that mean for their systems of communication, their cognition and the evolution of human language?
Studying animal behaviour (just like studying the cognitive abilities of human infants before they can speak) is extremely hard because of the danger of overinterpretation. To take an example from my Amazonian field research, consider the Amazonian horsefly. The bite of this nasty little creature hurts more than most because they (the females only) suck blood by lacerating the skin. What’s worse is that the locations of the bites itch for a good long time afterwards. Hiking through the jungle is almost always rewarded with multiple bites from these pests, along with their partners in crime, mosquitoes, wasps and smaller species of blood-sucking flies. One thing about horseflies, though, is that they seem to know where you are not looking!
On a certain level, it often seems as though Amazonian horseflies must have minds that can figure out human behaviour. While it is true that they seem to use a strategy for choosing a location on the body (clothes are no impediment as they can easily bite through denim jeans and cotton T-shirts) based on an interpretation of other animals’ behaviour, should one then say that horseflies have a plan for sucking blood that is based on interpretation of their victim’s perceptions? Doubtful.
An alternative explanation could simply be that the flies are genetically programmed to bite the relatively darker areas of a victim – the shaded part of their appendages. A shaded part of your body will also be one in which the visibility will be much reduced. People often anthropomorphise and interpret as cognitively designed actions what are in all likelihood physically determined.
Getting back to primates and animals more generally; there are many rigorous studies that avoid overinterpreting animals’ behaviours.5 One of the most problematic issues in the lengthy conversation in science about whether animals have cognitive abilities in any way similar to those of humans is the profoundly circular assumption that cognition requires language, human language at that, and that therefore animals cannot have cognition because they lack language. This is simply declaring by fiat that humans alone have cognition, before research has been conducted. Such ideas are misguided by their anthropocentric framing of the questions.
These views derive from the work of René Descartes in the seventeenth century, who believed that only humans ‘think therefore they are’. Descartes’s views arguably set back studies of human cognition because they discouraged comparative evolutionary studies of mentality. They also affected non-human studies by simply declaring that animals lacked mental lives.
In Descartes’s view, non-humans possess no consciousness, no thought and no feelings. Additionally, his view that human minds are disconnected from bodily experience led instinctively to his linguistic-based theory of cognition, namely that only language users think.
But as philosopher Paul Churchland aptly puts it: ‘Among many other defects, it [the account that only humans think because thinking requires language] denies any theoretical understanding whatever to nonhuman animals, since they do not traffic in sentential or propositional attitudes.’6
Any view of cognition that ignores non-human animals ignores evolution. Whether we are talking about the nature of ineffable knowledge or any other kind of cognitive or physical capacity, our account must be informed by and be applicable to comparative biology if it is to have any explanatory adequacy. Animal cognition helps understand the importance of evolutionary theory and comparative biology in the understanding of our own cognition. It also allows for tremendous insight into how the bodies of both humans and other animals are causally implicated in their cognition.
The main problem with disregarding animal cognition is that, in doing so, we are essentially disregarding what cognition might have been like among our ancestors before they got language. Their prelinguistic state was the cognitive foundation that language emerged from. If there is no cognition before language, à la Descartes and many others, the problem of understanding how language evolved becomes intractable.
Of course, there are those who claim that language did not evolve gradually, so we wouldn’t expect to find its roots in any other species. According to such researchers, the grammatical core of language ‘popped’ into being via a mutation, bringing forth a linguistic Prometheus whose X-Men genes spread quickly throughout the entire species.
On the other hand, there are those who work experimentally to address the question of whether primates have beliefs and desires and whether other primates are capable of ‘mind-reading’. For both questions the evidence so far answers tentatively ‘yes’ – there does seem to be some form of these abilities in other primates.
So humans may not be alone in the world of thinking and interpreting others. But if other primates, such as chimpanzees with their 275–450cm3 brains, are capable of reading the intentions of other creatures, as well as holding beliefs and desires, then surely the 500cm3-brained primates of the genus Australopithecus or the 950–1,400cm3-brain-sized species of Homo had even more well-developed powers of cognition and social understanding.
Animals and fossils strongly support the idea that humans got their unique abilities by baby steps. And our debt for this knowledge goes back to the fossil hunters. The painstaking work of collecting fossils and attempting to piece together cultural and anatomical evidence for the origins of our species takes physical fortitude – to withstand the heat, sweat, remoteness and occasional danger of palaeontological field research. It is a cut-throat, competitive enterprise at times, with mudslinging from all sides.
But in spite of the hard, painstaking field research of palaeontologists, on 1 January 1987 an article appeared in the journal Nature which threatened to wrest all the glory, power and science from the palaeoanthropologists and transfer it to lab-coat-wearing geneticists. The paper, ‘Mitochondrial DNA and Human Evolution’, co-authored by Rebecca L. Cann, Mark Stoneking and Allan Wilson, argued that genetic evidence clearly established that the DNA of all current Homo sapiens traces back to a single female’s mitochondrial DNA about 200,000 years ago in Africa.
This was a bombshell. Could it really be that three people in a comfortable laboratory put an end to the controversy surrounding the ‘recent out of Africa’ vs ‘multiregional’ hypotheses? To review, the former claimed that Homo sapiens originated in Africa and migrated out, replacing other Homo species across the globe. The latter suggested that all modern humans evolved in separate lineages from the various sites of Homo erectus around the world.
It turned out that the multiregional hypothesis was shown to be largely incorrect. When it first became public, the ‘Mitochondrial Eve’ theory was met by criticism from the proponents of the multiregional hypothesis, among others. But it has held up well to scrutiny and is now accepted widely by palaeoanthropologists, biologists and geneticists. The lab workers beat the field workers on this one.
Before one can grasp the significance of Mitochondrial Eve for language origins, however, it is necessary to review the science behind the conclusions. This is the theory that underlies the notion of a molecular clock on which the Mitochondrial Eve story is based. Originating sometime in the early 1960s and first published in a paper by Linus Pauling and Emile Zuckerkandl, the molecular clock idea came about after noticing that changes in amino acids across species are temporally constant. Thus, knowing the differences in amino acids between two species can tell when these species split from a constant ancestor.
As with most scientific discoveries, several people soon added to these ideas. Then in 1968 Motoo Kimura published a now famous article, ‘Evolutionary Rate at the Molecular Level’, in Nature. Kimura’s paper laid out the basic ideas of a ‘neutral theory of molecular evolution’. The neutral theory here is non-Darwinian, meaning that, rather than natural selection, Kimura placed the responsibility for most evolutionary change on genetic drift produced by random, neutral variations in organisms. Since these changes do not affect the survivability of an organism, it is able to pass on its genes normally to viable and fertile offspring.
Applying the molecular clock to mitochondrial DNA collected from humans around the world led to the proposal that all living Homo sapiens come from a single woman (called ‘Lucky Woman’ or ‘Mitochondrial Eve’) in Africa, about 200 millennia ago. In other words, only one woman from the past produced an unbroken line of daughters up until the present, thus transmitting her mitochondrial DNA to all living humans.
The genus Homo thus arose in Mother Africa. But if life was so good in Africa, why, when and how did our Homo ancestors leave there?
* Humans are able to run down game for several reasons. First, unlike any quadruped, humans are able to breathe hard while running. Second, humans’ lack of fur, their perspiration, and their upright posture (with its greater surface area exposed to evaporation of perspiration) allow them to cool far more efficiently than quadrupeds. A human running after a horse, other things being equal, will eventually catch it.