Australasian Science: Australia's authority on science since 1938

Evolution Goes Back to the Drawing Board

What if snakes or whales could regrow legs, or chickens develop teeth, or humans re-evolve tails like our primate ancestors? Reversible evolution is possible under certain conditions – even after many millions of years – according to a study published in Evolution (http://tinyurl.com/zkdj5hy).

An international team of scientists found that some of the largest kangaroos ever to evolve resurrected crests on their teeth that were present in their distant ancestors more than 20 million years earlier. They speculate that changes in climate, habitat and diet provided the selection pressures that resurrected these dental features. As forests retreated towards the coastline over millions of years, kangaroos were forced to eat more grass, and their teeth needed to cut rather than grind their food.

Biologists have often discounted the potential for evolution to shift into reverse, dismissing such occurrences as cases of convergent evolution. However, co-author A/Prof Gavin Prideaux of Flinders University argues that “reanimating genetically mothballed features may be ‘allowed’ by evolution when it aligns with pressures that determine an animal’s ecology”.

PhD candidate Aidan Couzens found that “small changes to a ‘rule’ that determines how teeth form in the embryo have allowed some kangaroos to partly turn back the clock on evolution. Using these rules, we can start to predict the pathways evolution can take.

“We found that, contrary to Dollo’s law in biology, features lost in evolution can re-evolve when evolution ‘tinkers’ with the way features are assembled in the embryo,” Couzens says.

Prideaux says that kangaroos and wallabies have been studied as barometers of historical climatic change in Australia. “They have been around for at least 30 million years,” he says. “We are discovering more about how early forms were adapted to the abundant soft-leaved forest plants, and how later macropods adapted to more arid conditions.”