A whiff of early brain evolution

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.

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‘Wasabi receptor’ is snake’s infrared sensor

SNAKES have a unique sensory system for detecting infrared radiation, with which they can visualize temperature changes within their immediate environment. Using this special sense, they can image the body heat radiating from warm-blooded animals nearby. This enables them to track their prey quickly and with great accuracy, even in the dark, and to target the most vulnerable parts of the prey’s body when they strike. It also warns them of the presence of predators, and may be used to find appropriate locations for building dens.

Infrared detection is known to be mediated by a specialized sensory apparatus called the pit organ, but several important questions about the detection mechanisms remain. It is still unclear, for example, where in the pit organ the infrared sensor is located, and whether it detects light particles directly, in a similar way to the eye, or heat energy. These questions have now been answered by a group of researchers from the University of California in San Francisco. In an advance online publication in the journal Nature, they report the identification of the sensor: it is an ancient protein called TRPA1, which has been adapted for this purpose in snakes, but not in other vertebrates.

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