A young lizard crawls over the eggs of its brethren. (Photo: Yin Qi)
The old riddle “Which came first, the chicken or the egg?” is not much of a riddle to biologists. The shelled amniote egg, which is familiar to many of us as chicken eggs, evolved about 325 million years ago. The wild ancestors of chickens, in contrast, only appeared a few million years ago (and chickens were domesticated much more recently than that). Among other birds, chickens evolved with shelled eggs, so the gradual transition to chickens occurred with very little change in the egg. In fact, the basic structure of amniote eggs is highly conserved across most species of amniotes, which include mammals, turtles, crocodilians, snakes, lizards, and birds.
The evolution of shelled eggs had a huge impact as it allowed amniotes to colonize land. The closest relatives of amniotes are amphibians (frogs, salamanders, and caecilians), and most amphibians also lay eggs. However, amphibian eggs are surrounded by jelly-like capsules that easily dry out if they are not in water, or at least in wet soil, which makes colonization in drier areas impossible. In contrast, many amniotes can lay eggs in very dry places where water is scarce. This freedom from water allowed mammals and reptiles (including birds) to move into many terrestrial environments where amphibians cannot live.
The amniote egg is more complex than many realize. In addition to the water-retaining shell, amniote eggs have four membranes that surround the developing embryo, and a yolk to provide nourishment to the embryo. It is essentially a complete “room and board” for the developing offspring, which can then exist and develop independently of the mother.
Given all the advantages of a shelled egg, why would a species evolve to produce young with any other method, such as live birth? When did this process of egg laying disappear in most mammals, including humans?
Hundreds of millions of years ago, an ancestral mammalian lineage evolved viviparity, or live birth. In viviparous mammals, the egg is retained and implanted in the mother’s uterus, the egg shell does not form, and a placenta forms that serves to supply nourishment and gas exchange between the mother and embryo. Except for the shell, the other membranes of an amniote egg still exist in mammals, although in modified form (see Figure).
A shelled amniote egg (left) compared to a developing embryo inside a viviparous mammal (right). Figure from Principles of Life, 3rd edition, by D. M. Hillis et al. (2019; https://macmillanlearning.com/Catalog/product/principlesoflife-thirdedition-hillis/studentresources), reproduced by permission of Oxford University Press. For permission to reuse this material, please visit http://global.oup.com/academic/rights.
Most people associate viviparity with mammals. But viviparity only evolved once in ancestral mammals, whereas it has evolved about 100 times in lizards and snakes. Viviparity is especially common in lizards and snakes that live at higher elevations. At high elevations, environmental temperatures are usually cooler, so eggs laid at high elevations develop very slowly. If the embryos can be retained inside the mother, then the female can raise the incubation temperature through thermoregulation (sitting in direct sunlight, for example). So viviparity has evolved repeatedly in many different groups of reptiles, often as an adaptation to life in cooler environments such as high elevations. Changes in the opposite direction (from viviparity back to oviparity, or egg-birth) are thought to be much rarer, in part because the gland that makes the egg-shell has been lost in viviparous species. However, the latter point has long been debated, and “re-evolution” of oviparity has been suggested in some groups of lizards and snakes.
A lizard gives live birth. (Photo: Wei-Wei Zhou)
A recent study by Gao et al. published in Proceedings of the National Academy of Sciences (https://www.pnas.org/content/early/2019/02/06/1816086116) examined the genetic changes that accompany a transition from oviparity to viviparity in closely related species of toad-headed lizards (Phrynocephalus), which live at various elevations around the Tibetan Plateau of central Asia. The authors found that many of the same genes are involved in the production of shelled eggs and viviparous embryos, but that they differ in the order and the magnitude that these genes are turned on and off. Thus, the transition between oviparity and viviparity appears to be largely a matter of changes in gene expression, rather than the evolution of new structural genes. In other words, genetic changes in gene regulation can result in major morphological and physiological adaptations in reproductive mode over relatively short periods of evolutionary time. This helps explain how there have been so many changes between oviparity and viviparity, and also suggest that changes in the reverse direction may not be as difficult to achieve as were once thought.
