Mouse to elephant in 24m generations

IT TAKES 24 million generations for a mouse-sized animal to evolve to the size of an elephant, scientists have shown.

For the first time researchers have measured how fast large-scale evolution can occur in mammals, and have examined the increases and decreases in mammal size following the extinction of the dinosaurs 65 million years ago.

The team looked at 28 mammal groups, including elephants, primates and whales, from various continents and ocean basins over the past 70 million years.

The findings show changes in whale size occurred at twice the rate of land mammals, thought to be because it is easier to grow big in the water because it supports the weight.

The international team of 20 biologists and palaeontologists also discovered that rates of size decrease are much faster than growth rates. It takes only 100,000 generations for very large decreases, leading to dwarfism, to occur.

Dr Alistair Evans, an evolutionary biologist at Monash University in Melbourne, Australia said the study was unique because most previous work had focused on micro-evolution, the small changes that occur within a species. “Instead we concentrated on large-scale changes in body size. We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change – a massive change, but also a very long time,” Dr Evans said.

“A less dramatic change, such as rabbit-sized to elephant-sized, takes ten million generations.”

The research furthers understanding of conditions that allow certain mammals to thrive and grow bigger and circumstances that slow the pace of increase and potentially contribute to extinction.

Many of the species that have shrunk, such as the dwarf mammoth and dwarf hippo eventually became extinct.

Dr Evans added: “The huge difference in rates for getting smaller and getting bigger is really astounding – we certainly never expected it could happen so fast.”

Dr Jessica Theodor, associate professor of biology at the University of Calgary, and co-author of the study, said: “Our research demonstrates, for the first time, a large-scale history of mammal life in terms of the pace of growth. This is significant because most research focuses on micro-evolution, which are changes that occur within a specific species.”

The research is published in the Proceedings of the National Academy of Sciences USA.