Which came first: the baby or the egg?

Saturday, August 23, 2014, 6:56 PM - Not the usual question, but an important one for evolutionary biologists.

A new study published in American Naturalist is taking a closer look at live birth and its benefits when compared to the egg-laying method common in birds and reptiles.

The earliest mammals also laid eggs. In fact, platypuses and echidnas still do so today. At some point, the transition was made towards live-birth. Fossil evidence suggests that the earliest live-birth mammals existed about 160 million years ago. But analyzing an evolutionary leap from so long ago is very difficult and so Oliver Griffith from the University of Sydney is taking a look at lizards.

While most reptiles lay eggs, there are examples of lizards that have evolved toward pregnancy—and this evolutionary step has come more recently than that of mammals. One Australian species has done this so recently that still has some populations that lay eggs. By comparing the live-bearing and the egg-laying populations of the same species, scientists are able to infer a lot of information.

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While pregnancy offers some benefits—eggs can't survive in cold environments—it also requires placenta which is a support organ for the fetus. To develop a placenta, organisms need a vast availability of food and it seems that makes all the difference. When food is scarce, some lizards abort and eat their offspring. While this behavior may seem odd to humans, it's further proof that an availability of resources may have been tied to the evolutionary leap.