The long and short of it is that things remain contentious (Image: Louie Psihoyos/Science Faction/SuperStock)

The common ancestor of modern mammals was tiny and shrewlike, living unobtrusively in the shadow of the dinosaurs – or so we thought. A genetic analysis now suggests it may have been more like a small monkey in size.

Fossils indicate that some larger mammals shared the dinosaurs’ world, but palaeontologists think that they all disappeared alongside the giant reptiles. Only tiny mammals survived, giving rise to all modern forms.

Nicolas Galtier of the Institute of Evolutionary Sciences in Montpellier, France, begs to differ. With colleagues, he used common features in the genomes of 36 modern mammals to sketch out the genome of the creature from which they descended.


Reconstructing the detailed genome is impossible, but Galtier managed to recover two of its properties. In modern mammals, these properties are correlated with body size and lifespan. Galtier’s results suggest the ancestor of modern mammals weighed at least a kilogram, and lived over 25 years’.

Michael Novacek of the American Museum of Natural History in New York is sceptical, given what the fossil record tells us. Several modern mammal groups such as rodents emerged after the dinosaur extinction, and the fossils show their first members were small. “There’s no question about that,” says Novacek.

But Galtier points out that the fossil record is incomplete. A large mammal ancestor that ultimately gave rise to all modern mammal groups, including the rodents, might simply have failed to fossilise.

Journal reference: Molecular Biology and Evolution, doi.org/jfk