Speciation is inevitably a slow process, not observable in a human lifetime outside a laboratory. Loss of species is also usually measured on geologic scales. Whenever the environment changes faster than can be accommodated by evolutionary organismal change, the affected organisms will likely go extinct. Environmental change includes introduction of new predation modes, temperature or chemical transitions, or any other habitat modification that is not survivable.
Rarely, in response to external catastrophes, extinction can become rapid or instantaneous and wide-spread. The history of life on Earth can thus be described as long boring periods of essential stasis, punctuated by sudden and random calamity. There have been five documented great extinctions in the history of life on earth. It bodes well for humanity that many of us can contemplate a sixth great extinction and work to prevent it as best we can.
We humans have observed rapid extinction of a species. Almost always in such cases, humans are the agent of the changes that doom a species, either by direct elimination of another life form (e.g. great auk), or indirectly, through disease, pollution, or destruction of environments required to support the species (e.g. many species of frogs), or both direct and indirect impacts (e.g passenger pigeon).
Since evolution is a background process defining life, species naturally go extinct all the time as nature maintains its sustainable balance. Why should we care if a few of us find financial, convenience, or primal motivation in disappearing a few others? Stanford’s Paul Ehrlich prophesizes: “In pushing other species to extinction, humanity is sawing off the limb on which it perches”.
To help us further understand the dynamics of accelerated extinctions, Elizabeth Kolbert has provided us detailed examples in her book The Sixth Extinction: An Unnatural History. This is not exactly news to most people. Every small child learns about extinctions while being told about dinosaurs. Dinosaurs seem magical creatures to a child, for they do not exist in our reality. But then they learn that they once did exist.
Although the concept of extinction is readily comprehensible by small children, it took scholars millennia to discover its nature and further generations to understand the process. Since we can only know of extinct species from their remains, realization of lost species had to await the discovery and analysis of physical evidence. In the modern day, lost species also can be inferred from gaps in our genetic tree of life.
The rest of this essay is based on Kolbert’s book, which begins with Georges Cuvier, the late 18th century French naturalist who first hypothesized species extinction. He explained that fossilized remains, of the American mastodon, the woolly mammoth, the giant sloth, and the Maastricht fossil, represented species that were lost forever. His hypothesis was all the more remarkable because he lacked an evolutionary perspective, Darwin having not yet been born. Extrapolating his few examples, he predicted there were entire worlds of lost species waiting to be discovered.
By 1800, Cuvier had expanded the list of extinct species from 4 to 23, most of which resembled living species. Among his analyses, though, was the correct identification of a skeletal anatomy wholly unfamiliar, which he called a wing-fingered reptile or ptero-dactyle. Cuvier published his research in 1812, by which time the number of identified extinct quadrupeds numbered 49.
Geologic stratigraphy was just recognizing that rock types are found in layers associated with different geologic periods. The placement of fossil remains in such layers led Cuvier to hypothesize that life has a direction. Life forms recorded near the surface resemble current living species in many respects, while remains at lower layers become more and more strange compared to living species, as mammals disappear and the previous world of dinosaurs is encountered.
Yet all these realizations did not lead Cuvier to hypothesize life’s evolution, or transformisme as it was then known. For he was an expert only at analysis of creatures who were the leaves on the tree of life at particular moments in history. There were no intermediate forms to study. And each extinct example seemed uniquely suited to a specific mode of existence. He could not imagine what purpose an intermediate form might realize.
His contemporary, Jean-Baptiste Lamarck, was champion of transformisme. He hypothesized that life transitioned continually to more complexity, pushed by some life force. He also recognized that change in environment resulted in concomitant change in organisms that lived in that environment. Birds adapting to live in water developed webbed feet. Moles moving underground lost need for eyesight.
Yet Lamarck did not recognize extinction as a valid hypothesis (except that he saw humans as a change agent capable of eliminating certain types of animal). He rather thought that animal fossils represented earlier forms that had subsequently been completely transformed into current species via transformisme.
Cuvier found Lamarck’s theories absurd, that animals could change their body types when convenient. His favorite counter-example was of mummified Egyptian cats, which prove indistinguishable from current cats even after thousands of intervening years. Lamarck called such an argument essentially short-sighted; a thousand years is a blink of the eye in the geologic time scale. Yet, Cuvier offered no hypothesis to explain the population of the world by different sets of animals in different eras.
Cuvier’s hypothesis for extinction was a cataclysmic event. When the number of known extinctions was small, one cataclysm was hypothesized. As the number of examples and their time frames grew, several cataclysms became necessary for the explanation. This led Cuvier to believe that like life, earth’s environmental state had a direction, from extreme mutability to a more steady state. This explained why no extinction-class cataclysms were recorded in our history or observed by living humans.
Cuvier’s observations supporting cataclysm have since been shown to be wrong. The changes noted were slow and continuous, not sudden or random. But his theory of cataclysms itself is now accepted. And in all things anatomical, his expertise led to correct explanations. Particularly cogent was his prediction of a megafauna extinction of six thousand years ago, taking out many of the larger quadrupeds including mastodon and mammoth. We now place this extinction at thirteen thousand years ago. It is hypothesized to be the first significant extinction assisted by homo sapiens. It is thus appropriate to call this the start of a new era of life on Earth, the anthropocene.
The analyses of ancient Earth processes continued in the next generation with geologist Charles Lyell. He became friendly with Cuvier, but denied his theories of cataclysm and life’s direction to ever more complexity. Lyell thought that extinctions must occur imperceptibly slowly. All indications to the contrary could be explained as the result of an incomplete data record.
Further, Lyell conjectured species once extinct could pop up again at a later time. All manner of creatures have existed in prior eras, and could exist in the present and future eras as well. With respect to geologic processes, presently observed processes are key to the past. He extrapolated this concept to living organisms as well.
In the early 1830s, Lyell published his three-volume Principles of Geology, the manifesto of the uniformitarian school of thought. It became one of the primary influences on the mind of young Charles Darwin. Volume One was presented to Darwin by the captain of the Beagle just before it commenced its five year voyage. It so impressed Darwin that he ordered Volumes Two and Three be sent ahead.
Darwin set out to confirm many 0f Lyell’s geologic theories. Examples of uplift, volcanism, and earthquakes were encountered that all reinforced Lyell’s hypotheses of how mountain ranges came to be. The more Darwin explored, the more Lyellian the world seemed. A Lyell biographer wrote “without Lyell there would have been no Darwin”. Darwin reciprocated “my books came half out of Lyell’s brains”.
How Lyell and Cuvier before him could reject outright the hypothesis of transmutation of life forms remains an intellectual mystery. Perhaps religious indoctrination played a role. Although the fossil record documents new life forms over and over again, neither man addressed how they originated. It fell to Darwin to explain the gradual adaptive change that happens to all life forms as they struggle for existence. Lyell’s gradual changes to our physical world may have served to inspire Darwin to realize the slow change at work in living organisms as well.
Darwin failed to develop a complete extinction hypothesis. He understood extinction as a gradual change in the numbers of a species as they failed to compete successfully for their right to exist. Darwin mocked Cuvier’s reliance on cataclysm to explain extinctions. Darwin further thought extinction must happen even more slowly than speciation, to explain the great abundance of existent species. Both processes were magnitudes too slow to be observed in a single human lifetime.
How quickly a species is likely to go to extinction is called its background extinction rate. It is calculated by observing the biological record of similar species. For example a mammal species is expected to go extinct every 700 years, based on 5500 mammal species and a heuristic background mammalian extinction rate of 0.25 per million species-years.
Given the slow background rates of extinction, such an event happening to a larger land animal is very unlikely to be witnessed in a human lifetime. Yet the extinction of the great auk happened in Darwin’s lifetime, as did that of a Galapagos tortoise and the dodo. These species went from multitudes to none. The auk extinction was widely reported in the English press.
Darwin never seemed to have sensed the contradiction of such events with his purely gradualist theory. Had he not disrespected Cuvier so much, he might have seized on catastrophe as a sometime accelerant for the extinction process.
The five prior great extinctions (loss of over half of living species) were global in extent, happened orders of magnitude more quickly than the background extinction rates of the species, and left our biosphere in a vastly different state. The greatest of the great extinctions was the end-Permian event. All current living species descend from the just 4% of species that made it through this Great Dying.
Prior mass extinctions all had different characteristics. Some of them happened in phases that spanned hundreds of thousands of years, and in one case, millions of years. Others happened in a blink of geological time. Multiple causes are hypothesized, including volcanism, continental drift, release of methane from methane clathrates, and extraterrestrial impact objects. To which we are now adding humans, with ever lessening contrary argument, as the evidence piles on. Richard Leaky has warned: “Homo sapiens might not only be the cause of the sixth extinction, but also risks being one of its victims”.
Examining our impact at a high level, humans over-hunt and fish, utilizing some species to the last individual; poach and slaughter the most endangered large animals, just to cut off small pieces and thus satisfy cultural fetishes; pollute the biosphere to wipe out species indiscriminately; cut up the environment into small pieces that cannot support the species who lived in the greater area before; through our global travel and transport, re-homogenize species to the effect of recreating a Pangaea-like environment of a single connected landmass. All of these actions result in impoverishment of global biological diversity, even though some local diversity may actually increase.
We have in our power to undo some of the species losses we are currently directly inflicting on our biosphere. We have the power to decide which evolutionary pathways will remain open on our watch. We need to choose wisely. Conservation and restoration will be necessary on a large scale. The indirect things we do that affect the entire biosphere’s health may be much harder to undo, and it is here that we are placing ourselves at greatest risk.
Today’s naysayers, who vote to block corrective measures, may be correct in calculating they have the resources required to cloister themselves and ride out the gathering storm in their lifetime. But as we approach the year 2100, their children’s children’s children will despair at the unfathomable hardship their ancestors’ arrogance, selfishness, and self-deception will have bequeathed. They will correctly surmise their naysaying forebears should have known better, given the evidence available to them, or at least should have listened to those who did.