Jules Howard (Elliott & Thompson)
What do you think of when you picture an egg? Almost certainly something oval 鈥� most likely a hen鈥檚 egg, at least at first. But eggs as a broader category exist in many forms and have been around almost since the dawn of multicellular life.
Yet, as science writer Jules Howard lays out at the start of Infinite Life: A revolutionary story of eggs, evolution and life on Earth, they rarely take centre stage in accounts of evolutionary history. In fact, the entire period between conception and birth barely gets a look in. Instead, the spotlight is fixed on tales of competition, predation and sexual selection, whose players are already off gallivanting around the world.
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That won鈥檛 do, says Howard. Taking us on a journey from 635 million years ago to the advent of Homo sapiens, he argues that eggs 鈥� whether large and shell-covered or minuscule and jelly-like 鈥� have been key to some of the most profound innovations in the history of life on Earth.
Among the many things that we fail to appreciate about eggs is their role in the move from water to land. Importance is usually placed on the development of robust limbs and on the capacity to breathe air. While both are clearly essential, so, too, is a tougher, hardier egg that can withstand exposure to the atmosphere without drying out, explains Howard.
This was evident during the Carboniferous Period some 350 million years ago, when insects and the lineage of vertebrates that would give rise to mammals and reptiles independently developed additional membranes around their eggs, allowing gases in and out while locking moisture inside.
The change empowered these animals to exploit new niches further away from rivers, ponds and swamps, resulting in an explosion of diversity. Without it, says Howard, all insects would be like silverfish 鈥� unable to live anywhere except where it is humid and damp 鈥� while all land vertebrates would be more closely tied to the spawning pool, much as amphibians are today.
The consequences of this shift are captured in his prose. For example, in order to leave its waxy egg, glued with 鈥渟urgical precision鈥� to the underside of a leaf, a young shield bug must pop open an 鈥渆scape hatch鈥� before clambering into the great beyond. The immensity of the project facing a baby vertebrate as it emerges from its egg is likened to leaving behind 鈥渁 locked room, for single occupancy, inside which is a well-stocked kitchen, a comfortable bed, good insulation and a chemical toilet鈥�.
I did find myself a little disappointed, however, that Howard didn鈥檛 go into more detail on the fossil evidence for landmark events in egg development. While brief mention is made of Alfred Romer, who discovered what was once thought to be the earliest amniotic egg 鈥� the kind developed by the ancestors of reptiles and mammals 鈥� more space could have been afforded to explaining how we know when this marvel evolved.
During later chapters, Howard鈥檚 scope broadens significantly as he considers the development of the larval life stage in insects, the relative merits of the marsupial鈥檚 pouch versus the placental mammal鈥檚 uterus, and how parental habits in watching over eggs may have played a role in the rise of the dinosaurs.
While this departure from egg anatomy can jar, it starts to make sense when you realise that each development is made possible only because of the egg. For example, in the case of marsupials and placental mammals like humans, the umbilical cord is derived from and extends the concept of the allantois, one of the membranes in amniotic eggs that helps the embryo exchange gases and handle waste.
Overall, Infinite Life achieves its goal of putting the egg front and centre in the story of evolution. Though it sometimes lacks depth, it makes up for this with an excellently told story covering a broad range of evolutionary time. If you are anything like me, the areas where it is sparse will simply compel you to find out more 鈥� which makes it an excellent primer for new ways of thinking about the story of life on Earth.
