Chimpanzees shed light on the evolution of human senses 19/3/04. By John Pickrell Insights into the evolution of sight, smell and speech are emerging from comparisons of the human genome with the DNA of chimpanzees and other primates.

Humans share up to 99.2 per cent of our genetic make-up with our closest relative, the chimpanzee, although it has been difficult to pinpoint the genetic factors that define the human species. Now with the newly drafted chimp genome sequence available (released publicly on 10 December 2003), researchers can start to search for those genetic differences that might explain what makes us uniquely human – and results are already beginning to emerge.

Comparing humans and chimpanzees

A study of thousands of human and chimp genes, published in the journal Science in December 2003, shows that there could be key differences in our perceptions of the world. Different suites of genes may govern how chimps and humans smell and hear, and differences in hearing might even have affected the evolution of speech, wrote Andrew Clark (Cornell University, Ithaca, New York) and colleagues at other research centres.

To find key differences between a selection of 7645 chimp and human genes, Clark and colleagues looked for evidence of positive selection.

Genes that had higher mutation rates than the expected background rate for each species (termed positive selection) were thought likely to have offered a survival advantage. These genes are likely to account for biologically significant differences, the kind that might define humanness.

Clark's team found differences in genes for cell signalling and amino acid metabolism, but some of the most interesting evolutionary changes appear to have occurred in genes involved in human hearing and olfaction (or smell).

Three out of 21 hearing genes tested showed high levels of positive selection in the human lineage. Clark and his co-workers speculate that the understanding of spoken language in humans may have required fine-tuning of hearing ability. The gene most affected by selection is called alpha tectorin, and plays a vital role in the membrane of the inner ear. "We were surprised to see that many genes involved in hearing came out as showing significant signs of positive selection," said Clark.

"Altering the properties of the tectorial membrane would affect its vibrational properties, like gluing blocks of wood or drilling holes in the sound board of a violin."

(Although Clark is not sure this is related to the development of speech, individuals with tectorial membrane mutations do have inherited hearing deficits making it hard to understand speech. The FOXP2 gene, which underwent positive selection in the human lineage, has also been linked to speech.)

Of the 48 olfactory genes tested, 27 human genes showed much positive selection since humans and chimps diverged approximately five million years ago. The results are surprising, because human do not appear to rely strongly on smell to survive.

Differences in chimp and human lifestyle are likely to be behind the change, said Clark. "Finding food and sensing whether it is edible or noxious, [and] attraction to mates," are possible selective pressures driving the evolution of human olfaction,” he speculated.

From smell to sight

Another study, detailed in the January 2004 edition of the journal Public Library of Science Biology, argues that, perhaps like the victim of a sense-robbing disability, humans and other apes may have sacrificed some sense of smell to develop full-colour vision.

Humans, mice and the other great apes all have a similar number of olfactory receptor genes. Yet up to 60 per cent of these are inactive in man, compared to only 30 to 40 per cent in great apes and 20 per cent in mice and dogs.

"There has been a decrease in the size of the intact [olfactory gene] repertoire in apes relative to other mammals, with further deterioration in humans," wrote evolutionary biologists Svante Paabo, Yoav Gilad and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Gilad's team found evidence that a higher proportion of olfactory pseudogenes may be linked to the evolution of trichromatic or full-colour vision in some primates. Humans, apes and Old World monkeys (baboons and macaques, for example) use three different proteins called opsins in their eyes that detect wavelengths of light in the red, green or blue parts of the spectrum. In contrast, most New World monkeys (such as marmosets) only have two colour detecting opsins and have a much-limited range of colour vision.

By comparing the proportion of olfactory pseudogenes across primates, the researchers showed that Old World monkeys and apes have a higher proportion than their New World counterparts, with one exception. All New World monkeys have 20 per cent or less olfactory pseudogenes, apart from the howler monkey, which has more than 30 per cent psuedogenes. Strikingly, that species has independently developed full-colour trichromatic vision.

The development of full-colour vision - and reliance on vision - may have relaxed the need for a sensitive sense of smell, argue the researchers. Further studies of what smell and sight related cues are involved in selection of food and mates may confirm that idea, they suggest.

Clark believes that it's quite possible that we experience smell, sound and other sense in a different way to chimpanzees. "It is important to realise that seeing rapid evolution in some proteins in sensory perception does not really tell us much about the actual sensation of those signals," he said. "Just like a blind human develops especially acute hearing, many factors go into tuning the senses for one's needs."

Image credit: Matthew Herring

Further reading

Clark AG et al. Inferring Nonneutral Evolution from Human-Chimp-Mouse Orthologous Gene Trios. Science 2003; 302: 1960–1963. Abstract

Gilad Y, et al. Loss of Olfactory Receptor Genes Coincides with the Acquistition of Full Trichromatic Vision in Primates. PLoS Biology2004; 2: 0120-0125. Abstract ; Full text