One of the most distinctive characteristics of Homo sapiens is our exceptionally large brain, and enhanced cognitive capabilities. In fact, large brains, measured by the encephalisation quotient (EQ), are a characteristic of primates in general, and brain size and complexity has been increasing in the primate lineage for nearly 70 million years. However, this trend is particularly noticeable in the human lineage, and the last 3 million years of hominid evolution have seen the most pronounced increases in encephalisation, with a tripling in brain size. Such a rapid increase in size is extraordinary, especially for an organ so complex. Some areas of the brain have expanded disproportionately, such as the cerebral cortex, which has increased in size by 3 orders of magnitude since our divergence from Chimpanzees. The cerebral cortex accounts for around 85% of total brain volume in humans, and is responsible for complex mental functions.
The Genetics of Brain Evolution
It seems likely that human brain evolution was contributed to by mutations in a large number of genes; each producing small, incremental changes. Furthermore, many of the differences between humans and primates may be due to changes in gene regulation. There is little evidence of positive selection in recent evolutionary history in genes expressed in the brain. However, regulatory regions for genes involved in brain function and development show much greater evidence of recent evolution.
Our understanding of the genetics behind human brain evolution has also been aided through the study of abnormality; Microcephaly is a condition which causes a dramatic reduction in brain size. It is especially pertenant because the condition only affects brain size. Mutations in six genes are associated with Microcephaly. Two of these genes, Microcephalin and ASPM, have been studied intensely, and show strong evidence of positive natural selection during human evolution. One variant of Microcephalin is believed to have arisen around 37,000 years ago, coinciding with the appearance of modern human behaviour. ASPM >has been under positive selection for much longer, perhaps even since the beginning of brain expansion in primates.
Some scientists believe that the human brain also has much higher gene expression levels than other primates, and consequently, more brain activity. Changes in gene expression are coded for not within genes but in the regulatory regions that surround them. A study of the regulatory region of 6,000 genes found evidence for recent evolution in 250, a significant proportion of which are involved in brain development and function.
Genes acting outside the brain may have also been crucial for increases in brain size. For example, jaw muscle size has a strong influence craniofacial morphology and therefore may limit brain size. MYH16 is a gene which is thought to have reduced the size of jaw muscles during human evolution, enabling increases in brain size.
What was the cause of the continued selection for larger brains and increased intelligence during primate and human evolution? One popular explanation known as the social intelligence hypothesis, suggests that the most complex challenge faced by an a animal is interacting with other members of its species, especially forming long-term relationships. Supporters point out that intelligence is most often associated with more sociable lifestyles. High levels of intelligence have also been found in other apes, some birds and cetaceans such as dolphins. While these species are sociable, there are also many highly social examples whose intelligence is not noteworthy.
Brain size evolution was also influenced by more logistical concerns; child birth. As our ancestors left the trees and moved towards a bipedal posture, this necessitated changes in the shape of the pelvis, making the birth canal narrower and more convoluted. An adaptation that allowed these two conflicting changes to coexist was the fontanelles; openings in the skull which enable the skull to compress slightly during childbirth. These openings remain for several years of life, enabling massive increases in brain size that occur at this stage. Recent fossil evidence shows that the appearance of delayed fontanelle closure coincided with our move to a bipedal stance, around the time of the Australopithecines. In Chimpanzees, brain development is almost totally complete at birth, and the fontanelles are closed. In order to achieve larger brain sizes, humans we forced to have more altricial, needy young. We simply cannot grow our brains any larger prior to birth, or childbirth itself would be impossible.
How unique is our intelligence?
The encephalisation quotient (EQ) allows comparison of brain size between species, taking into account differences in body size. It is the ratio of brain size to body mass that really counts, as most of the brain is required for maintaining bodily functions and thus a higher brain-body ratio equates to a greater amount of available brain that can be utilised for other functions. Bottlenose dolphins have a brain mass of around 1600g, slightly larger than humans (1350g) and several times larger than the Chimpanzee (400g). However, their EQ is lower than humans (4 in dolphins compared to 7 in humans).
|Species||Brain Size||EQ||% Brain Development in Womb|
|Human||1300 – 1400 g||7.44||28%|
|Dolphin||1500 – 1700 g||4.14||42.5%|
Primates are generally quite intelligent, and several species are able to make tools both to obtain food or for social displays. They have complex social structures with strong hierarchies, and are capable of high levels of cooperation. Some species can be manipulative and deceptive and most can recognise friends and enemies. Primates are also highly communicative and can learn and understand aspects of human language. Although controversial, some scientists believe that Chimpanzees, unlike other primates, possess a concept of self. In mirror tests, chimpanzees will attempt to remove a coloured sticker from their face when they see it in the mirror, indicating that they understand that the image presented to them is themselves. In both chimpanzees and orangutans (Pongo abelii) there is evidence of forethought and planning, suggesting that advanced cognitive abilities may have evolved earlier than previously thought.
Elephants are, again, highly social and also very long-lived. These two factors may have driven the evolution of intelligence in elephants. Certainly, their long-lasting social bonds give elephants the opportunity to demonstrate a wide variety of highly intelligent, and touchingly human behaviours such as grief, compassion, art, humour, self-awareness and tool-use.
Birds, in particular members of the Corvidae family (Crows, ravens, rooks, jackdaws, jays, magpies and nutcrackers), also show exceptionally high intelligence, and their brains are the same relative size as chimpanzees. Corvids live in large, complex social groups, and fledge much later than most bird species, enabling them a lengthy developmental period to learn. They have been shown to exhibit remarkable navigation and long-term memory capabilities, as well has having practical intelligence in the form of tool use. New Caledonian Crows can use a stick to gain access to food out of reach at the bottom of a plastic tube, and Rooks have demonstrated problem solving abilities; using pebbles to raise the water level and obtain a reward.
Corvids also exhibit social intelligence. Western Scrub-jays hide food by burying it. Knowing that their stored food may be vulnerable to theft, scrub-jays who are aware that they have been watched burying something will often return later to move the item and rebury it in private. This suggests that Western Scrub-jays are capable of empathy and forethought. Self-awareness, as measured by an animal’s response to being shown its own face in a mirror, has been demonstrated in European Magpies, among others.
Moving away from terrestrial habitats, dolphins (members of the order Cetacea), are well-known to be highly intelligent. Dolphins, like many of the other intelligent animals, are highly social and have complex systems for communication with other members of their species. They are also believed to have individual recognition, and have shown creativity and humour. Evidence also suggests that, like Chimpanzees and Elephants, dolphins are self-aware. As they use echo-location as their primary sense, the dolphin’s brain stem is much faster than that of humans, enabling them to process information from sound, which travels faster in water than air. Dolphins may also have some basic numerical skills.
Another sea-dweller, but a much more distant relative of humans, the octopus, is also unusually intelligent. Octopuses are molluscs (along with squid, snails, mussels, and a range of shelled sea-creatures) in the order cephalopoda. However, unlike other molluscs, cephalopods are active predators, and the need to locate and capture prey may have been the driving force behind the evolution of their intelligence. In the subclass Coleoidea (cuttlefish, squid and octopuses), extrodinary levels of cooperation and communication in hunting have been observed, and in some species classical conditioning and observational learning have been reported. Cephalopods are also capable of remarkable dexterity, a skill essential for tool use, and tool use has been widely reported in octopuses. Veined octopuses have been found to collect coconut shells, transport them, manipulate them and reassemble them into a shelter. Both octopuses and the more evolutionarily ancient Nautiluses have been shown to exhibit a long-term memory of days, even weeks. Cephalopod intelligence is fascinating because it is achieved by a nervous system totally different from that of vertebrates, however their intelligence does not rival that of vertebrates such as birds and apes.
One consequence of intelligence may be schizophrenia. Its prevalence remains high in the population (1%) even though it is both harmful and heritable and so would be expected to be removed by natural selection. A predisposition to schizophrenia may be coded in the same genes which were selected for causing increased intelligence; over a third of known schizophrenia-linked genes show signs of recent positive selection.
A more favourable and universal consequence of increased intelligence is language. There is evidence to suggest that brain size was a prerequisite for the evolution of language. There are a number of genes which have been implicated in the evolution of the human brain and language. FOXP2 is involved in speech and language, and the appearance of two mutations in this gene, estimated to have emerged around 200,000 years ago, coincided with the appearance of Homo sapiens.
Although we often presume so, language is not unique to humans. Chimpanzees have the capacity to understand and learn human language; Nim Chimpsky (a chimpanzee) was taught sign-language and learned over 100 words in his lifetime. Bird song is widely appreciated to serve a communicative function, and birds have also been shown to use body language. Bird song has a major learned component, and some birds show local variation in song ‘dialects’, which are passed down through generations. Whales and dolphins also communicate using song, and dolphins are believed to have names; specific signature whistles that identify an individual. Interestingly, dolphins have been found to impersonate one-another, emitting each other’s signature call!
Intelligence is often considered to be a human speciality. But it is actually not particularly uncommon in the animal kingdom. And it is by no means a gradual assent towards man as some pinnacle. Arguably, some species such as Dolphins and Elephants may even be more intelligent than us. Language is a more restricted privilege, but vocal communication is not uncommon and several non-human animals are capable of using language to some extent. When it comes to language, however, humans have clearly taken it up-a-notch, and no other animal can use language including tone, intonation, grammar and gesture to the extent that we can.
Articles in this Series:
- Intro: What Makes Us Human?
- Part One: A Brief History
- Part Two: Intelligence and Language
- Part Three: Anatomical Adaptations
- Part Four: Culture and Faith
Want to know more?
- Bond and Woods (2006) Cytoskeletal genes regulating brain size Current Opinion in Cell Biology 18, 95 – 101
- Bamshad and Wooding (2003) Signatures of Natural Selection in the Human Genome Nature Reviews Genetics 4, 99 – 111
- Carroll (2003) Genetics and the making of Homo sapiens Nature 422, 849 – 857
- Casci (2005) Brains under pressure Nature Reviews Genetics 6, 822
- Dorus et al (2004) Accelerated Evolution of Nervous System Genes in the Origin of Homo sapiens Cell 119, 1027 – 1040
- Adam Eyre-Walker (2006) The Genomic Rate of Adaptive Evolution Trends in Ecology and Evolution 21 (10) 569 – 575
- Evans et al (2004) Reconstructing the evolutionary history of microcephalin, a gene controlling human brain size Human Molecular Genetics 13, 1139 – 1145
- Evans et al (2005)Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans Science 309, 1717 – 1720
- Evans et al (2006) Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage Proceedings of the National Acacdemy of Science 103, 18178 – 18183
- Evans, Vallender and Lahn (2006) Molecular evolution of the brain size regulator genes CDK5RAP and CENPJ Gene 375 75–79
- Gilbert, Dobyns and Lahn (2005) Genetic links between brain development and brain evolution Nature Reviews Genetics 6, 581 – 590
- Hill and Walsh (2005) Molecular insights into human brain evolution Nature 437, 64 – 67
- Nielsen et al (2005) A Scan for Positively Selected Genes in the Genomes of Humans and Chimpanzees PloS Biology 3, 976 – 985
- Sabeti et al (2006) Positive Natural Selection in the Human Lineage Science 312, 1614 – 1620
- Vallender and Lahn (2004) Positive selection on the human genome Human Molecular Genetics 13, R245 – R254
- Crespi, Summers & Dorus (2007) Adaptive evolution of genes underlying schizophrenia Proceedings of the Royal Society B, 274; 2801 – 2810
- Clayton, Dally & Emery (2007) Social cognition by food-caching corvids. The western scrub-jay as a natural psychologist Philosophical Transactions of the Royal Society B, 362; 507 – 522
- Emery (2006) Cognitive ornithology: the evolution of avian intelligence Philosophical transactions of the royal society B, 361; 23 – 43
- Emery et al (2007) Cognitive adaptations of social bonding in birds Philisophical transactions of the royal society B, 362; 489 – 505
- Hof and Van der Gucht (2007) Structure of the cerebral cortex of the humpback whale, Megaptera novaeangliae (Cetacea, Mysticeti, Balaenopteridae) Anat Rec (Hoboken) 290 (1): 1–31
- Macphail (1982) Brain and Intelligence in Vertebrates Oxford University Press
- Falk D et al (2012) Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution PNAS
- Lonsdorf et al (2009) An experimental, comparative investigation of tool use in chimpanzees and gorillas
- Jones and Kamil (1973) Tool-Making and Tool-Using in the Northern Blue Jay Papers in Behaviour and Biological Sciences 66
- Montevecchi (1978) Corvids using objects to displace gulls from nests Condor 80
- Osvath and Osvath (2008) Chimpanzee and orangutan forethought: self-control and pre-experience in the face of future tool use Animal Cognition 11
- Bugnyar (2008) Animal Cognition: Rooks team up to solve a problem Current Biology 18
- Reiss and Marino (2001) Mirror self-recognition in the bottlenose dolphin: A case of cognitive convergence Proceedings of the National Academy of Sciences of the United States of America 98 (10): 5937–42
- Bird and Emery (2009) Rooks Use Stones to Raise the Water Level to Reach a Floating Worm Current Biology 19: 1410 – 1414