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.
Nervous tissue is extremely fragile, and so is very well protected. The brain, which has a jelly-like consistency, is encased in the skull, and is surrounded by cerebrospinal fluid, which acts to cushion it against blows that might cause it to come into contact with the inside of its bony case. Likewise, the spinal cord is surrounded by the vertebrae, the series of bones which runs down from the base of the skull.
Being so soft, the brain and spinal cord decompose quickly. When an animal dies, the nervous system begins to disintegrate immediately, until the armour in which it was enveloped is all that remains. Thus, the organ is rarely, if ever, preserved, and brain fossilization is considered to be impossible – or so it was thought.
Now though, a team of French and American researchers report that they have found what appears to be an intact brain in a fossil specimen of Sibyrhynchus denisoni (above), a long extinct relative of the shark and ratfish which lived some 300 million years ago. Their findings are published online later this week in Proceedings of the National Academy of Sciences.