Date:
September 3, 2014
Source:
Field Museum
Most living mammals are active at night (or nocturnal), and many other mammal species are active during twilight conditions. It has long been thought that the transition to nocturnality occurred at about the same time as mammals evolved, around 200 million years ago. This thinking was based on on features such as the large brains of mammals (good for processing information from senses like hearing, touch, and smell) and the details of light-sensitive chemicals in the eyes of mammals.
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