paleontology11: Is a sixth mass extinction event in progress? (v1.0)

 This is an extract from CNN and a good epilogue for the paleontology series. 

No species lasts forever — extinction is part of the evolution of life. But at least five times, a biological catastrophe has engulfed the planet, killing off the vast majority of species from water and land over a relatively short geological interval. The last of the 5 mass extinction events was 66 million years ago. 

A growing number of scientists believe a sixth mass extinction event of a magnitude equal to the prior five has been unfolding for the past 10,000 years as humans have made their mark around the globe. This extinction is caused not by a city-size space rock like the fifth extinction event but by the overgrowth and transformative behavior of a single species — Homo sapiens. Humans have destroyed habitats and unleashed a climate crisis. Calculations in a September study published in the journal PNAS have suggested that groups of related animal species are disappearing at a rate 35% higher than the normally expected rate. And while every mass extinction has winners and losers, there is no reason to assume that human beings in this case would be among the survivors. In fact, study coauthor Gerardo Ceballos thinks the opposite could come to pass, with the sixth mass extinction transforming the whole biosphere - or the area of the world hospitable to life - possibly into a state in which it may be impossible for humanity to persist unless dramatic action is taken. “Biodiversity will recover but the winners (are) very difficult to predict. Many of the losers in these past mass extinctions were incredibly successful groups,” said Ceballos, a senior researcher at the Institute of Ecology at the National Autonomous University of Mexico.

The dodo, the Tasmanian Tiger, the baiji, or Yangtze River dolphin, and the Western black Rhino are just a few of the species that have disappeared so far in what’s known as the Holocene or Anthropocene extinction. While the loss of even one species is devastating, Ceballos of the National Autonomous University of Mexico has highlighted that the ongoing episode of extinction is mutilating much thicker branches of the tree of life, a metaphor and model that groups living entities and maps their evolutionary relationships. Entire categories of related species, or genera, are disappearing, a process he said is affecting whole ecosystems and endangering the survival of our own species. 

Ceballos and his study coauthor Paul Ehrlich, Bing Professor Emeritus of Population Studies at Stanford University, assessed 5,400 genera of vertebrate animals, excluding fishes. A single genus groups one or more different but related species — for example the genus Canis includes wolves, dogs, coyotes and jackals. The duo’s analysis found that 73 genera had gone extinct in the past 500 years. This is much faster than the expected “background” extinction rate, or the rate at which species would naturally die off without outside influence — in the absence of human beings, these 73 genera would have taken 18,000 years to vanish, the researchers said. The causes of these extinctions are varied — land-use change, habitat loss, deforestation, intensive farming and agriculture, invasive species, overhunting and the climate crisis — but all these devastating changes have a common thread: humanity.

Ceballos pointed to the extinction of the passenger pigeon, which was the only species in its genus, as an example of how losing a genus can have a cascading effect on a wider ecosystem. The bird’s loss, a result of reckless hunting in the 19th century, narrowed human diets in eastern North America and allowed the bacteria-harboring White-footed mice that were among its prey to thrive. What’s more, some scientists believe the passenger pigeon’s extinction, combined with other factors, is behind today's tick-borne diseases such as Lyme disease that plague humans and animals alike, according to the study. Not only do the destructive actions of humans have the potential to erode our quality of life in the long term, but their ripple effects could eventually upend our success as a species, according to Ceballos.

While branches of the tree of life are vanishing, the distribution of certain animal species is becoming more homogenized — the world is home to about 19.6 billion chickens, 980 million pigs and 1.4 billion cattle. In some cases, intensive farming can trigger outbreaks of disease like avian influenza outbreaks that rip through poultry farms and increase risk of spillover in wild migratory birds. Other farm animals act as hosts for virus that infect humans, with the potential to cause pandemics like Covid-19. 

Ultimately, the planet can and will survive just fine without us, Ceballos added. But what might the final traces of human civilization look like in the geological record?