This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists studying one of nature's greatest reproductive innovations have discovered how some lizards evolved from laying eggs to giving birth to live young. The new research is the first to reveal the evolutionary process behind this important biological change and offers a unique insight into how other species—including mammals—may have evolved to give birth to live young millions of years ago.
The study, led by researchers at the University of Glasgow and published in Current Biology, reconstructs how live birth evolved in the Eurasian common lizard. Researchers found that the reproductive change occurred not through one large evolutionary leap, as some scientists had previously assumed, but through the gradual accumulation of many small genetic changes over time. The findings have implications for our wider understanding of reproductive evolution and reveal never-before-seen information on how small, incremental genetic changes can lead to major biological transformations.
The transition from laying eggs to giving birth to live young is one of the most significant innovations in animal evolution and has happened multiple times in most vertebrate lineages, including fish, amphibians, reptiles and mammals. However, because most transitions to live birth occurred tens or even hundreds of millions of years ago (as in mammals), scientists have struggled to understand the evolutionary process that first gave rise to this complex reproductive strategy. Although lizards and humans reproduce in different ways, the structures involved in carrying and nourishing developing young have shared evolutionary origins.
The egg sac around a baby in a live-bearing lizard is evolutionarily comparable to the membranes and placenta that surround and support a developing human baby during pregnancy. The common lizard provides a rare opportunity to study this process because it contains both egg-laying and live-bearing lineages that diverged relatively recently in evolutionary time. In the new study, the researchers analyzed whole-genome sequences from lizards sampled across the species' range.
They then used these data to reconstruct how genetic differences accumulated during evolution and gave rise to live birth. The analyses showed that, rather than arising through one or two major genetic changes, the transition resulted from natural selection acting on many regions across the genome over a long evolutionary timescale. The changes occurred primarily in regulatory DNA that controls when and where genes are switched on during pregnancy.
Consistent with this regulatory mechanism, genes in these regions were more highly expressed in the uterus of pregnant live-bearing lizards, suggesting that changes in gene regulation played a central role in the evolution of live birth. The findings provide one of the clearest reconstructions yet of how a complex evolutionary innovation emerges. Hongxin Xie from the University of Glasgow's School of Biodiversity, One Health and Veterinary Medicine said, "Live birth is one of the most remarkable evolutionary innovations among vertebrates, but because it evolved so long ago in most animals, we've had very little opportunity to understand how it actually arose.
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