A Walk Through Time

Night Journeys

First Mammals

225 Million Years Ago

Small and nocturnal, the first mammals jump, climb, swing and swim through the dinosaur world. Obliged to inhabit small niches in a world of giants, mammals will discover that their diminutive size opens a proverbial window of opportunity.

In the first waves of mammal diversification, some rodent-sized insect-eaters evolve lactation, enabling mothers to spend more time in the nest keeping their young both fed and warm. Some mammal species evolve even smaller bodies.

These insect-eating mammals, among the oldest known, grew to about 4 inches (10 cm) in length. (Illustration by Zdenek Burian, © Jiri Hochman and Martin Hochman)

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