By Josef Eisinger ©
In 1930, during the early days of radio broadcasting, Albert Einstein admonished a radio audience in Berlin not to consume the fruits of science with as little intellectual curiosity as a cow chewing its cud devotes to botany. Since that time almost a century of astonishing scientific and technological advances have come to pass and I wonder if a comment similar to Einstein’s could not be made today regarding the general public’s attitude toward science and its achievements.
I, however, am one of those who relish belonging to the first generation that knows the age of our universe and its history—even the physical laws that govern it—though gaps remain; and I am thrilled to belong to one of the earliest generations that can be aware of the checkered past of our planet and, specifically, of how life evolved on it. It is a story that was sketched with remarkable insight by Charles Darwin; but subsequent scientific advances, particularly radioactivity-based dating of fossils and sophisticated DNA sequence analysis, have provided us with a far more detailed account of the heritage of Earth’s species, including our own.
To envisage a vast span of time like the 4.54 billion years of our planet’s existence it is helpful to suppose that Earth was created just one year ago, on January 1st, and to look back on the year’s events from the vantage point of midnight on December 31. This allows the evolution of life to be displayed in its proper perspective. In that make-believe time domain, it took until the middle of January (or about 200 million years) for the newly formed planet to cool sufficiently for its surface to retain liquid water—the essential solvent of all life as we know it. There exists strong fossil evidence that by the beginning of February single-celled life forms (prokaryotes, e.g. bacteria) were already thriving in Earth’s oceans, although the antecedents of these early denizens of the ocean are not known and are subject to various hypotheses. They had plenty of time (some two billion years) to evolve in complexity.
The DNA analysis of a large number of today’s species suggests that LUCA (last universal common ancestor) was alive in March. The genome of this hypothetical organism included the 355 genes that every living thing on Earth has in common with it, be it a microbe, a tree, a flower, or an elephant. Knowing the functions of the proteins that these 355 genes encode, we can even paint a crude portrait of LUCA: it was a single-cell anaerobic microbe whose DNA, composed of the same four bases as ours, floated freely in its cytoplasm—which was enclosed in a lipid bilayer membrane, as in a bacterium. Some of LUCA’s genes suggest that it lived in a high temperature environment, not unlike those near deep sea vents.
Sometime in August an event occurred that eventually altered all future life in a dramatic fashion: certain bacteria incorporated, within their cell walls, a pigmented protein complex (now known as the photosynthetic reaction center or PRC) which was capable of converting radiant energy into useful chemical energy and, in the process, generated oxygen molecules. Sustained by the abundant sunlight on Earth these photosynthesizing bacteria flourished and, in time, generated enough oxygen to constitute one fifth of Earth’s atmosphere. Since oxygen was poisonous to most of the then-existing life forms the so-called Great Oxygenation Event (also called the Oxygen Catastrophe) led to the extinction of numerous species. The bacterial PRC that accomplished this remarkable feat is, apart from cosmetic differences, identical to the PRCs that inhabit green leaves and sustain plants.
Early in September another development took place that had far-reaching consequences for life on Earth. Some of the single-cell organisms acquired a membrane-enclosed nucleus in which its genetic information was conveniently packaged. This innovation of eukaryotes, as these nucleated cells are called, provided many advantages, the most important one being that it facilitated the emergence of multi-cellular life forms whose individual cells shared a common genetic master plan. All through the fall a vast number of multi-cellular species proliferated in what is known as the Cambrian explosion and by December 1st the first vertebrates had evolved; fishes and tetrapods populated the oceans, and the first land plants made their appearance.
Then, on December 10th (251.4 million years ago) disaster struck. The so-called Permian-Triassic extinction event caused the demise of some 90 percent of the species dwelling in the oceans, possibly because something caused Earth’s temperature to rise and thereby deprived its ocean-dwelling creatures of their life-sustaining dissolved oxygen.
But life went on. Beginning in mid-December, numerous species of dinosaurs evolved and roamed over Earth’s land and seas for some two hundred million years—until disaster struck once again. On December 26th (66 million years ago) another extinction event occurred, possibly because of the impact of a large meteorite. It wiped out 50 percent of all species including all the dinosaurs except the avian ones whose descendants—we call them birds—remain with us still.
During the last five quiescent days of our retrospective year a great array of new species evolved, including mammals—those warm-blooded animals that had acquired the knack of bearing live offspring. The fossil record tells us that by December 29th (20 million years ago) many familiar mammals—bears, giraffes, hyenas, and apes—populated Earth’s forests while Homo habilis and several other humanoid species were living some four hours ago. And about half an hour ago modern humans, optimistically named Homo sapiens, made the scene in Africa and, before long, populated all the continents.
Earth’s most recent catastrophic event was the last ice age during which a kilometer-thick sheet of ice covered large areas of the Northern Hemisphere. When the ice melted, a mere 90 seconds ago (i.e. 12,000 years ago) it caused the oceans to rise dramatically and ushered in Earth’s present Holocene period. This is the era in which humans invented agriculture and viniculture, discovered how to smelt metals, invented the art of writing, built great cities, and forged vast empires; Julius Caesar was assassinated just fifteen seconds ago. Modern science began to bloom about two seconds ago (ca. 18th century) and, in time, allowed humans to discover the bumpy history of our planet and the astonishing story of how life evolved on it. That is unquestionably an intellectual achievement that deserves to be celebrated—if Earth was not, even now, careening toward another extinction event. This one, unlike the many that preceded it, is of our own making and must be attributed to the inability of humans to co-operate in curbing climate change—while there is still time.
Josef Eisinger is a physicist and molecular biologist, and is professor emeritus at the Mount Sinai School of Medicine in New York. Born in Vienna, he is the author of over 150 scholarly articles ranging from nuclear physics to the history of science. He is also the author of “Einstein on the Road” which is based on Einstein’s travel diaries, and “Einstein at Home” which draws on the recollections of the Einstein family’s housekeeper in Berlin (Prometheus Books 2011, 2016), as well as Eisinger’s own memoir “Flight and Refuge: Reminiscences of a Motley Youth” (Amazon 2016).