From simple blob to spaceship, Jules Howard explores how the egg cracked open the secrets of evolution
By Katie Burton
The egg (an organic vessel grown by an individual to carry offspring to term) is an extraordinary thing, and has been so for millions of years. In Infinite Life, wildlife expert Jules Howard charts the egg’s story, or rather, all the different permutations of egg that have existed, moving from the very beginnings of life on Earth to the present day.
Because this story is told chronologically, moving through the different geological eras chapter by chapter, I wondered at first if this was just another step-by-step retelling of life on Earth, with the egg simply a new way to position the story. Quickly, however, I realised I was wrong. Howard centres eggs in his telling, revealing that across many times and places, adaptations and changes to eggs are at least part of the reason for the very success (or failure) or certain organisms. In doing so, he raises a number of fascinating questions and adds new theories about evolution to more established ones, lending credence to his thesis that eggs are all too often overlooked as vital agents of change.
Take sex for example. What’s the point of it, asks Howard? Why would organisms used to reproducing asexually bother going to the trouble of finding a mate, producing eggs and producing sperm? And, why have so many organisms, ourselves included, gone down the path of separating body cells from egg cells very early on in embryo development, rather than using stem cells to create eggs later on? After all, organisms that do the latter, such as jellyfish, salamanders and newts, have remarkable abilities. They can regrow limbs, eyes, even portions of their brains should they lose them. These questions and more are covered in Infinite Life and are all caught up in the story of the egg and all the infinitesimal changes that have moulded life on Earth as a result of unthinking natural selection.
It all begins, of course, in water. It was in water, during the Ediacaran Period (635 to 538.8 million years ago), that structures we could truly call eggs first began to fall upon sea floors, encasing life. Then, during the Cambrian explosion, some of these egg-producing creatures began to attach their eggs to sturdy structures, or even store them in pouches (‘the first protective womb’). Slowly, slowly, eggs became more polished, securing greater defences against the elements, perfecting their ‘siren song for sperm’ – chemical signals drawing sperm near.
During the Devonian Period (419.2 to 358.9 mya), for the first time, the eggs of some fish were housed within the female body. Meanwhile, other fish began an epic move (though one played out across minute adaptations, step by tiny step) – moving onto land. The ancestors of one such lineage would become amphibians, reptiles and mammals. Why did these fish do this? The established theories pay little attention to eggs, but surely, posits Howard, the presence of predator-safe puddles and pools within which to store eggs was a lure? It certainly seems convincing.
From here, another vital development: the amniotic egg. Involving a series of fluid-filled membranes – including the amnion to protect the embryo, the yolk-sac to feed it, and the chorion, an overall enclosure – this innovation essentially ‘land-proofed’ eggs. The amniotic egg is all important; it changed everything.
There follows all sorts of intriguing developments: the very first grubs and larvae begin reaching adulthood through metamorphosis (probably the result of a chance early hatching that worked out rather well), and the first parasitic insects start laying their eggs within other creatures. And, of course, there are the dinosaurs, whose eggs, it was discovered in 2015, were many-hued, just like so many bird eggs today. Howard sets out the many theories for this oddity, one of the most recent and compelling being that eggs are different colours and patterns to help parents identify and stay one step ahead of impostor species (such as cuckoos).
And then, finally, it’s on to mammals, which perhaps surprisingly, really began to thrive during the Jurassic, a time when mammals that had placentas, had pouches and which laid eggs (the precursors to modern-day platypus and echidna) were all present. It was, writes Howard, ‘an era of great mammalian expression and flexibility’, driven and demonstrated by changes to eggs. From these small mammalian creatures, scurrying about the feet of the giant reptiles, we move swiftly through the giants of the Pleistocene and finally to our own ancestors and the development of the human placenta, described (to the surprise of no one whose ever harboured one) as the most ‘energy-sapping placenta the world has ever seen’.
It is a fascinating journey, told lyrically and employing all the different ways of saying ‘egg’ the author could presumably think of. There’s the resting cyst, which evolved in cyanobacteria between one and two billion years ago, described as an ‘armoured sleepsuit within which organisms could see out hard times’, or the ‘miasma of blobs, slime and semi-symmetrical splurge’ of the Ediacaran era. It is, for a story about such a seemingly small thing, dynamically and beautifully told.
Howard concludes with a brief chapter on the ways eggs are still changing today, as a result of rapid climate change. It is a sobering end. In the UK, butterflies and moths hatch up to six days earlier than they did ten years ago. Aphids hatch a month earlier than they did half a century ago. The results of these changes can be detrimental to the animals themselves, but also to us. The eggs of blood-sucking ticks for example, are no longer killed in winter, leading to a three-fold increase in Lyme disease in 25 years. The structure of eggs is also changing. Studies have shown a reduction in eggshell thickness in blackbirds, song thrushes and mistle thrushes, all linked to acidification, mostly through pollutants. ‘The truth is,’ writes Howard, ‘that, for many species, there may not be time for the egg to adapt to environmental changes so sharp and jagged as this.’ And, if eggs can’t adapt, nothing can.
The egg is a beautiful thing, far from simple and far from static. If you’ve never given it much thought before, this book will change that.
