When Kangaroos Came Down from the Trees

by Rileypie

Kangaroos are marsupials, that unique group of mammals that originated in the Americas, then crossed Antarctica 55 million years ago to end up in Australia by around 25 million years ago. Although kangaroos do not belong to a single species, but to several related species in the family Macropodidae, the group is characterized in the mind of the public as all having disproportionately large hind legs and a propensity to hop along the savannas that it inhabits.

But people today would probably not even recognize the ancestors of these iconic mammals, which were opossum-like critters that spent their lives up in the trees and first ate fruits and then tree leaves. The prevailing view at one time was that the kangaroos did not move down from the trees until around 15-5 million years ago, when growing aridity in Australia caused them to gradually evolve to have the attributes we see in them today.

Modern kangaroos loiter about in their grassland home (by By AWS10)

But a new study disputes that theory.

Researchers studied the teeth of more than 1600 kangaroo specimens, both from modern kangaroos and their fossil ancestors. They focused on the crown height and wear on the teeth, which gave them an indication of what the animals ate. Animals that eat relatively soft foods such as leaves and fruit do not have high crowned teeth, and the wear on the teeth is much less than if the animal ate something abrasive such as grass. This is because grasses are filled with silica bodies called phytoliths, which make eating them akin to eating sand, and the high crowned teeth is an adaptation towards eating such tough food.

Modern Tree Marsupial, who remained in the trees, by By Fred Hsu

When these researchers looked at the changes in teeth from the ancestors of kangaroos to their modern counterparts, they found that the change in dentition (from low crowned to high crowned teeth) and explosive radiation of kangaroo species did not happen as the land first grew arid (15-5 million years ago). Instead, this change only happened 3 million years ago, when C4 grasses started pushing back the trees and forests to create the grasslands and savannas that are now so prevalent in that continent.

This tight correlation between the expansion of grasslands in Australia and the evolution of the kangaroo as we know it today again highlights the importance of our old growth grasslands to the rise of so many iconic and magnificent animals. 

In fact, without the grasslands, our own ancestors would not have done the same thing as the kangaroos did, and we might still be lounging lazily in the treetops today, our minds bereft of the keen intelligence that (allegedly) is a hallmark of Homo sapiens.

Reference:

Couzens AMC, Prideaux GJ. Rapid Pliocene adaptive radiation of modern kangaroos. Science. 2018 Oct 5;362(6410):72-75. doi: 10.1126/science.aas8788. PMID: 30287658.