The birthing room: Reptiles switch, then switch again!

Courtesy Alex Pyron, George Washington University

Biology ain’t what it used to be. Once, scientists hoped to identify a single line of human ancestry, but a tangled web has emerged. They thought dinosaurs went extinct — when they actually evolved into birds. They thought life was impossible near water’s boiling point and that genes are “destiny.”

Wrong, wrong and wrong.

This week, we read another doomed theory, one that highlights the new-found primacy of gene regulation over the genes themselves. This one involves the order Squamata, the 10,000-odd reptile species with scaly skin, including lizards, snakes and amphibians. We were surprised to learn that about 2,000 squamates have a live birth, or vivipary, instead of laying eggs.

Scientists who traced the origin of live birth among Squamata thought it had evolved separately among 115 sub-groups of the order. This week, a large study of genetic relationships among Squamata claims vivipary originated 175 million years ago in an ancestral squamate.

Study authors R. Alexander Pyron, an assistant professor of biology at George Washington University, and Frank Burbrink did not have fossil evidence for live birth in the first ancestor. Instead, they relied on a genetic tree they created to show relationships among the squamates.

But once Squamata started its own branch of the evolutionary tree with live birth, many members of the order switched to egg-laying. And then some of them pulled a second switcheroo, and returned to live birth.

Mapping it out

So the conventional wisdom takes another dive. “People had assumed that these 115 groups had evolved live birth independently, but our major conclusion is” that a live-birth ancestor started the Squamata, says Pyron.

In other words, at the beginning, all squamates received the genetic equipment needed for live birth.

Paleontologists had already found fossils of ancient reptiles that showed that live birth has ancient roots, Pyron says. “There have been a number of recent discoveries of live births in squamates and close relatives, from the Cretaceous, around 100 million years ago, consisting of a fossil with the appearance of a fully developed embryo inside the [body] structure, without obvious, calcified eggs.”

When Pyron and Burbrink built a taxonomic tree showing the relationships among the squamates, they noticed that some species shuttled back and forth between live birth and eggs. For example, after the sub-order Iguania (iguanas, chameleons and some lizards), evolved from the early, viviparous squamates, they laid eggs. Later, a couple of dozen member of Iguania returned to live birth.

Near Calitzdorp, Western Cape, Little Karoo, South Africa, 27 September 2006; Martin Heigan

Baby, it’s too cold outside!

Pyron says the evolutionary choice is likely determined by environmental conditions — especially temperature. The “classic assumption” is that live birth evolved where eggs might freeze or fail to develop. Reptiles produce little body heat, but they can use “behavioral thermoregulation” — a fancy term for lounging in the sun — to stay warm.

And that seems to be the case for the squamates, Pyron says. “Groups that have recently moved to cold areas, like lizards in Norway, have renewed viviparity.”

But mothers that give birth must devote more nutrition to their offspring, and their increased weight makes them prone to predation. So the ideal reproductive solution can change depending on temperature and the presence of food — and predators.

Mechanically speaking: one final question

We’ve been taught that eggs and live birth are completely different strategies, but the switching we’ve discussed shows they are much closer than that. “When they switch from egg to live birth, it seems relatively simple, hold the egg inside, and the egg shell is not deposited,” says Pyron. “When they move in the reverse direction, the amniotic sac around the egg is calcified.”

The larger point is that genes can go silent for a time and then reactivate, Pyron says. “There are instances, such as the loss of eyes in cave animals, where genes are turned on or off; it’s more about regulatory control than about genes.”

By revealing patterns in evolution, a “tree of life” can be a powerful tool for answering evolutionary questions, Pyron says. “With squamates, the question that Frank and I tossed around was, how does viviparity evolve? We went into it with the classic assumption that it evolved 115 times. We were blown away by this novel result, that it originated much earlier; and it would have been impossible to know this without a large-scale tree of life.”

– David J. Tenenbaum