THE question of how mammals evolved their exceptionally large brains has intrigued researchers for years, and although many ideas have been put forward, none has provided a clear answer. Now a team of palaeontologists suggests that the mammalian brain evolved in three distinct stages, the first of which was driven by an improvement in the sense of smell. Their evidence, published in tomorrow’s issue of Science, comes from two fossilized skulls, each measuring little more than 1cm in length.
Mammals emerged during, or just before, the early Jurassic period, some 200 million years ago. We know that the earliest mammals were small, nocturnal animals that fed on insects, but there is very little in the way of details about how their brains might have looked, because fossils are scant, consisting mostly of isolated jaws and teeth. A few skulls have been found but until now studying the brain involved damaging the fossils which, given their rarity, was out of the question.
Zhe-Xi Luo, curator and associate director for research and collections at the Carnegie Museum of Natural History, found one of these rare skulls, along with the rest of the fossilized skeleton, about 10 years ago in the Yunnan province of China. It belonged to a tiny weasel-like creature that lived about 190 million years ago. The animal probably weighed just 2 grams, but it’s skull was conspicuously large relative to it’s 3 cm-long body. Luo therefore named it Hadrocodium, meaning “big head”.
Closer examination of the skull revealed several other features that are characteristic of mammals. The bones of the middle ear were separated from those of the lower jaw, suggesting that the animal had a well developed sense of hearing, and the lower jaw contained cone-shaped teeth similar to those of other early insectivorous mammals. Significantly, Hadrocodium is much older than all other known early mammals – it pushes the emergence of these mamalian features back by some 45 million years, and may have been the ancestor of all living mammals.
Having studied the external features of the Hadrocium skull for about a decade, Luo and his colleagues exploited recent advances in medical imaging techniques to look inside without damaging it.
In the new study, they used high resolution computed X-ray tomography to scan the skull and generate an endocast of the cavities inside it (above). The cast closely approximates the size and shape of the brain, enabling the researchers to digitally reconstruct the organ. They did the same thing with the skull of another early mammal called Morganucodon, and then compared both endocasts with those of more than 12 other animal fossils and more than 200 living animal species. (All of the scans used in the study have been uploaded to the DigiMorph.org website.)
The digital reconstructions provide anatomical details of the brains once contained within the skulls, which in turn give us clues about how the animals might have behaved. One of the researchers’ main observations was that the nasal cavities of Hadrocodium and Morganucodon are nearly 50% larger than those of the mammal-like reptiles called cynodonts. Furthermore, the olfactory bulbs are clearly delineated from the brain proper by a deep fissure. They also observed enlargement of the olfactory region of the brain, and of the somatosensory cortex, which processes touch information from the skin surface.
Thus, expansion of the olfactory bulb and olfactory and somatosensory cortices account for much of the increase in brain size as the mammals evolved from reptiles. This led the authors to suggest a sequence of evolutionary events that drove the early expansion of the mammalian brain.
They conclude that the initial expansion in brain size was driven by an improvement in the sense of smell. It was also driven by an increased sensitivity to touch from the pelt of soft fur that covered the early mammals’ small bodies, and by improved motor co-ordination. And the jaw bone rearrangements that accompanied the subsequent emergence of different tooth types – canines and molars – freed up more space in the nasal cavity, allowing for further enlargement of the olfactory bulb.