Evolution in action: Scientists discover lizards on verge of leap from egg-laying to live births
Scientists have caught the process of evolution in action as a species of Australian lizard abandons egg-laying for live births.
The variety of skink, which is snake-like with four tiny legs, has been found laying eggs along the coast of New South Wales.
However, the same yellow-bellied three-toed lizard living in the colder mountainous region is giving birth to offspring like a mammal does.
There are only two other types of modern reptiles which use both types of reproduction methods – another skink species and a European lizard.
One in five snakes and lizards gives birth to live young, with records showing nearly a hundred reptile lineages have changed from egg-laying in the past.
Study co-author James Stewart, a biologist at East Tennessee State University, in America, told National Geographic that the discovery provided scientists with a rare opportunity.
‘By studying differences among populations that are in different stages of this process, you can begin to put together what looks like the transition from one [birth style] to the other,’ he said.
Mr Stewart said the transformation could be linked to how newborns get nourishment. Or it could be a way of protecting the young in harsher climates.
Baby mammals are fed via a placenta which is connects the foetus to the ovary wall.
Through this it can breathe and pass back waste.
Embryos of egg-laying species get nutrients from the yolk while absorbing calcium from the porous shell, which also protects them from the external environment.
However, some fish and reptiles are using a mix of both birthing styles.
Mothers form an egg which she keeps inside her body until the last stages. The shells thin, allowing the embryos to breathe until birth – but, according to scientists, this poses a nourishment problem, as it contains less calcium.
This discovery prompted Mr Stewart and his colleagues to investigate the nutrient issue in the structure and the chemistry of the Australian lizard’s uterus.
He explained: ‘Now we can see that the uterus secretes calcium that becomes incorporated into the embryo – it’s basically the early stages of the evolution of a placenta in reptiles.’
However, Mr Stewart added that the process of reptiles moving from egg-laying to live birth is common in historic terms as making the switch is relatively easy.
‘We tend to think of this as a very complex transition,’ he added. ‘But it’s looking like it might be much simpler is some cases than we thought.’a