Human evolution
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Human evolution is the portion of evolutionary biology concerned with the phylogeny of the genus Homo. Specifically, it is the investigation of how Homo sapiens came to be a separate species, usually beginning from the last common ancestor of both humans and the extant great apes. However, investigations into the origins of other species of the Homo genus are also part of human evolutionary biology. This investigation heavily relies on the discipline of paleoanthropology, but also genetics, linguistics, sociology and psychology.
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History of Human Evolutionary Biology
For almost a century after Darwin, most views on human origins posited a linear progression (orthogenesis) from an ape to modern humans. This is often illustrated as a march of hominids (see picture at right), usually starting with chimpanzees, which become more human-like as they progress across the page (usually left to right). This view echoed the religious ideas of the time, namely the Great Chain of Being (a ladder of forms, with humans as the top of the physical forms, just below the angels and God). Even great evolutionary scientists, such as Thomas Huxley, were of the opinion that humans were a very special type of animal, somehow the pinnacle of biological evolution.
This anthropocentric view began to diminish as paleoanthropologists found it difficult to find fossils that fit within this progressive lineage. During the 1950s and 1960s, fossil finds of primates and humans led to the conclusion that humans did not evolve from an extant ape species, but both apes and humans evolved independently from a common ancestor (which would be classified as an ape). Indeed, independently divergent evolution became a central view throughout evolutionary theory, and scientists began to appreciate the many speciation events that led to humans. No longer was it assumed, as Theodosius Dobzhansky and Ernst Mayr had proposed, that only one species of hominin could exist at once. Molecular genetics confirmed the view of cladogenesis and multiple extinction events, rather than anagenetic human origins.
Derived Characters of Humans
A number of characters possessed by humans are unique among other apes, and many are unique to life in general. However, the majority of these characters were possessed by many primitive hominoids, but humans are the only one of those currently alive.
Terrestriality
Terrestriality, or living on the land (as opposed to an arborial lifestyle – living in the trees) is thought to have pre-dated bipedalism somewhat. However, there is no perfect consensus among scientists on this, with some paleoanthropologists considering an arboreal origin for bipedalism, so it possible that this is not the case.
Regardless, because of the fact that some extant apes, such as gorillas, are completely terrestrial and others, such as both species of chimpanzee, spend a significant amount of time on the ground, the evolution of terrestriality is not seen as a major adaptation and consequently is less researched in comparison to other areas of human evolution.
Bipedalism
Although Homo sapiens is not the only extant primate to walk on two feet, it is the only one to do so habitually and to have a striding gait (contrast with the waddling gait of chimpanzees) while doing so. The current scientific consensus on the origins bipedalism is that it occurred before brain development and culture, but after terrestriality (quadrapedalism is generally considered more efficient for an arboreal lifestyle, and it is estimated that 60% or more of time must be spent on the ground for selection towards bipedalism). It is generally placed to have evolved between 4.5 and 2 mya.
A number of anatomical adaptations are associated with bipedalism, including:
- a curved lower spine
- a shorter, broad pelvis
- a femur that slopes inward to the knee
- lengthened lower limbs
- an extensible knee joint
- a foot in which the first digit (big toe) is in line with the others
- a foramen magnum located more anteriorly, towards the center of the cranium (to support the new skull-neck orientation).
Though it may seem unlikely that all these transformations could evolve, it must be noted that many apes can walk upright, and even more can stand upright. Thus, evolution of bipedalism is an augmentation of an already present characteristic.
Many hypotheses have been proposed as to the selective pressure that lead to this development. They include an increased energy efficiency for endurance, improved avoidance of predators (such as being able to see over long grass), fruit-picking (thus requiring standing to pick berries), increased regulation of heat (an upright figure presents less surface to the sun during the middle of the day) and increased tool usage (where the hands would be needed to carry them), and many others.
While no clear consensus exists, likely the foremost explanation is the first listed: energetic advantages of bipedalism. During the late Miocene, drought slowly reduced the continuous African woodland of the middle Miocene to thinly dispersed patches of forest. The pressure to be able to efficiently transverse the savannah between these patches, to gain access to food resources and shelter, would have provided the impetus to bipedal adaptations. Although humanoid bipedalism is overall less efficient that quadrupedal locomotion (e.g. antelope), the action of walking is the exception, as bipedalism is slightly more efficient. Secondly, the knuckle-walking action of primates is certainly less efficient than any human gait, being roughly twice as energy intensive as conventional quadrapedalism. Thus, any adaptation towards bipedalism would give a selective advantage, provided that a chief pressure was increasing locomotor efficiency. However, some researchers, such as Karen Steudel, doubt that the intermediate bipedal gait would have been more efficient than knuckle-walking.
Growth of Brains (Encephalization)
Humans have the largest brain, in comparison to our body size, of any animal. It is mentionable that mammals have larger brains than any other vertebrate, and primates (and Cetaceans) have larger brains than any other mammal. Thus, being encephalization evidently precedes the evolution of humans. However, the brain of a modern human averages a volume of 1350cm3, whereas that of Australopithecus afarensis is estimated to have been approximately 400cm3 (comparable to modern chimpanzees). Thus, there was evidently a threefold increase in brain size over a period of 3 million years, whereas most other mammals did not experience any significant change during this time.
Having a large brain is not usually advantageous. The brain costs a lot of energy to maintain – consuming almost 20% of energy in an adult human, but only constituting 2% of body mass. Additionally, as the brain size of a newborn increases to become larger than the pelvic outlet (the principle limit on the size of the birth canal) of an adult female, selection will force the majority of development to occur postnatally (after birth). This results in much greater helplessness of a newborn, requiring a greater allocation of time and resources to care for offspring. Other problems, such as blood flow regulation and heat maintenance, are also created.
One might then ask what the selection pressures were that overpowered the disadvantages mentioned above to drive this rapid expansion in brain size. It was generally agreed that increased use of tools associated with a creative and intelligent brain were the likely cause of this evolution – in evolutionary artworks, modern humans were often portrayed holding a tool of some description. The benefits of tool use included the addition of large quantities of meat (a rich energy source to fuel the increased brain size) and control over the environment by means of fire and shelters. However, additional theories, not necessarily intended to supplant the former but to augment it, have been provided by primate researchers considering the social lives of primates. For an ape, use of tools and memorisation of food sources is probably nowhere near as complex as predicating and manipulating the behaviour of others in the group. Research has shown that a much larger neocortex is associated with large group sizes in primates[1]. The associated protection and cooperation of working as a group may provide the pressure to form such groups, and this pressure then acts on brain size to keep up with group size – those more able to manipulate the group will have more mating opportunities. However, brain size cannot increase forever, and will eventually hit a limitation, such as energy cost, that counteracts any future benefit of increased brain size.
Language
Communication is not unusual among animals, and most higher primates can produce a large range of sounds, which they use to produce different reactions in others. Chimpanzees appear to give difference distress calls for different threats and predators. Despite this, human language also encompasses grammar and syntax, and mere increase in primate vocalisation capacity would not have led to the development of modern language without a rapid increase in intelligence as well.
Unlike many other features of humanity, language leaves little trace for future generations, but can be detected indirectly as it is a key feature of culture. That is not to say, however, that language leaves no trace in the fossil record. Indeed, there are two key biological indicators of the capacity for human-like vocalisation. Firstly, the interior surface of a hominid skull can hint towards the capacity for that brain to process language (Wernicke’s area) and control the lips and vocal cords (Broca’s area). Using this methodology, there are indications of language as far back as Homo rudolfensis. Secondly, the larynx of an adult human is much lower in the neck than that of infants and all other mammals , yielding much room above the vocal cords for modification of resonating sound. However, this configuration tremendously increases the risk of choking when inhaling and swallowing at the same time (most other mammals will not choke while doing this). Thus, by looking at the larynx, and the base of the cranium, linguist Philip Lieberman and anthropologist Jeffry Laitman determined that australopithecines had a relatively ape-like laryngeal conformation, and by Homo erectus (basicraniums of Homo rudolfensis are poorly represented in the fossil record) the larynx had begun to descend into the neck, approximately as far as that of an 8-year old human[2]. The fully human laryngeal conformation is only found 300,000 years ago, in Homo sapiens.
Oddly, Homo neanderthalensis appears to have a flatter basicranium than Homo sapiens around the same period, hinting that Neanderthals could not have fully articulate human speech. It is possible that this is an adaptation for the Ice Age climate of Europe. However, other fossil find indicate that the musculature of the mouth may have compensated for this.
Culture
The evolution of intelligence and the development of languages brought with it another development: culture. Human culture includes a broad spectrum of human behaviours, such as art, technology, religion and commerce. The defining characteristic of culture in anthropology is the means of transmission; culture is learnt from others, rather than instinctive. Using this definition, it is clear that chimpanzees have elements of culture like social traditions[3].
The evolution of cultures is often comparable to the evolution of biological features, and evolutionary biologist Richard Dawkins used the term “meme” to refer to an idea that is transmitted, mutated and selected, just like genes are. The instinctive, biological basis of cultural behaviours is also a field of study, namely evolutionary psychology.
References
- Lewin, Roger & Robert A. Foley. (2004) Principles of human evolution Blackwell Science Ltd. Second edition. pp3-26, 240-253, 447-458
- ↑ Dunbar, R. (1992) “Neocortex size as a constraint on group size in primates.” Journal of Human Evolution 22:469-93
- ↑ Lieberman, P., Laitman, J. T. et al (1992) “The anatomy, physiology, acoustics and perception of speech: essential elements in analysis of the evolution of human speech” Journal of Human Evolution 23:447-67
- ↑ Whiten, A., Goodall, J., McGrew, W. C., Nishida, T., Reynolds V., et al (1999). “Cultures in chimpanzees.” Nature 399:682-5
