Iterations of evolution
SCIENCE • 11 Nov 2021 • Vol 374, Issue 6569 • p. 828 • DOI: 10.1126/science.abm0121
ITAI YANAI AND MARTIN J. LERCHER
A (Very) Short History of Life on Earth Henry Gee St. Martin’s Press, 2021. 288 pp.
With dramatic flair, Henry Gee’s sweeping new book, A (Very) Short History of Life on Earth, tells the 4-billion-year story of life on this planet and how it has been repeatedly shaped by geological, climatic, and atmospheric forces. Trained as a paleontologist, Gee tells life’s history using the framework of the fossil record, offering insights from the related fields of ecology and physiology. Interwoven as it is with geology and climate, life evolves the way Ernest Hemingway said we go broke: “gradually and then suddenly” (1).
Life emerged on Earth not long after the planet’s aggregation, writes Gee, and faced its first major challenge about 2.4 billion years ago. Until this point, bacteria and archaea had been confined to the oceans, where they evaded the Sun’s deadly rays, which were not yet tempered by a protective atmosphere. Bacteria eventually learned to harness sunlight to produce energy, with oxygen as a by-product; but as oxygen levels rose, generations of bacteria and archaea that had evolved in its absence were burned alive.
Later, continental collisions and volcanic eruptions caused waves of extinctions that wiped ancient species from the face of the Earth and propelled previously insignificant lineages into suddenly emptied ecological niches. The book returns often to this theme, reminding readers how creatures that dominated certain periods of Earth’s history—the dinosaurs of the Jurassic, for example, or the mammals of the Cenozoic—spent millions of years as minor players before taking center stage. Life, it seems, cannot help but exploit each new crisis it encounters as an opportunity for diversification. In this way, it is akin to the mythical phoenix, which is regularly consumed in a fiery inferno, only to rise again from the ashes.
The main protagonists of Gee’s book are vertebrate life-forms. Early vertebrates, which began as fish, evolved jaws, the better to shred other organisms with. Even after they began moving onto land, they still returned to the water to reproduce. Vertebrate embryos, notes Gee, need a liquid medium in which to develop. Thus, the most crucial vertebrate adaptation may have been the egg, which provides a liquid capsule in which life can unfold on dry land.
Another amazing early vertebrate adaptation was the development of air sacs, which first arose in dinosaurs and are still found in birds. This adaptation, which enabled a one-way system of air flow, also doubled as an efficient cooling system for internal organs.
As forests became more and more fragmented owing to climate changes linked to continental drift, primates started to venture into the open grasslands, from where the earliest hominins arose 7 million years ago. Their bipedal stance, notes Gee, made them “almost preternaturally maneuverable.”
About 2.5 million years ago, Homo erectus arose, a territorial savannah predator, deadly thanks to two traits: it was a powerful long-distance runner and a social animal. From this lineage, Homo sapiens evolved. Humanity’s first attempt at worldwide dispersal failed, shattered by the cold of an ice age 200,000 years ago. Confined to an oasis in what is now the Kalahari Desert, humankind nearly went extinct. We, as a species, are just as fragile as all the others, reminds Gee.
With beautiful exposition, Gee describes the major anatomical, physiological, and behavioral transitions in life’s evolution. He largely refrains, however, from discussing the mechanisms and agents of evolution, and he does not offer much evidence from fields other than geology and paleontology. With few references to genes and genomes, the book lacks an appreciation of the mechanisms by which the genes—the ultimate replicators (2, 3)—both constrain morphological change and enable incredible diversifications.
Gee’s unbridled excitement also prompts him to sacrifice some precision and downplay some scientific uncertainties in the interest of storytelling. He frequently puts forward matter-of-fact description of topics that are far from settled and weaves in his own “fanciful speculations,” freely admitting to these liberties in the endnotes. He advances a conjectural internal anatomy of Saccorhytus, a progenitor of vertebrates, for example; speculates about the reproductive strategies of ancient amphibians; and articulates how Stone Age shamans might have performed coming-of-age rituals.
Some hundreds of million years from now, Earth will become uninhabitable to even the hardiest organ isms, spelling the final doom for Earth-evolved life—unless, perhaps, some earthlings manage to escape into space first. Meanwhile, the reader is rewarded with a deeper appreciation of our own place in the grand scheme of life, where even the best-adapted species disappear within a time that is minute on the scale of evolution.
References and Notes
1 E. Hemingway, The Sun Also Rises (Scribner’s, 1926).
2 R. Dawkins, The Selfish Gene (Oxford Univ. Press, 1976).
3 I. Yanai, M. Lercher, The Society of Genes (Harvard Univ. Press, 2016).