The Tree of Life Is Losing Entire Limbs, Jeopardizing Evolution

Darren Orf
Mon, September 25, 2023 

Humans are Chopping Limbs Off the Tree of LifeMitchell Pettigrew - Getty Images

• Human-induced climate change and environmental degradation is causing entire genera to go extinct at rates 35 times higher than what’s been estimated to have occurred in the past one million years.

• A new study from Stanford University and the National Autonomous University of Mexico (UNAM) estimates that 73 genera, each representing many species, have gone extinct since the 1500s.

• This is a devastating blow for biological diversity and only reinforces the need to protect natural spaces—especially tropical rainforests, where an estimate 80 percent of known species reside.

In Charles Darwin’s world-changing work On the Origin of Species, the naturalist make use of the millennia-old metaphor in which all life is represented by a tree. He writes that “the affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth.”

But in this era of climate change and a human-induced sixth mass extinction, that tree has seen better days. A new study by Stanford University and the National Autonomous University of Mexico (UNAM) shows that not only are individual species blinking out of existence at an alarming rate, entire genera—representing branches on the tree of life—are being completely obliterated at a rate that hasn’t been seen on Earth for at least a million years. The scientists behind the paper concluded that we are witnessing a “mutilation of the tree of life.” The findings were published this week in the journal Proceedings of the National Academy of Sciences (PNAS).

“In the long term, we’re putting a big dent in the evolution of life on the planet,” Gerardo Ceballos, UNAM and study co-author, said in a press statement. “But also, in this century, what we’re doing to the tree of life will cause a lot of suffering for humanity.”

Using improved data from the International Union for the Conservation of Nature (IUCN), which tracks the conservation of status of all species, Ceballos and Stanford University’s Paul Ehrlich examined 34,600 species. These species represented a total of 5,400 genera of land-dwelling vertebrates.

Their findings show that 73 entire genera, each representing several species, have been completely wiped out since 1,500 CE. The hardest hit group was birds, with a total of 44 genera extinctions, followed by mammals, amphibians, and then reptiles. That’s a genus extinction rate 35 times higher than what’s been estimated to have occurred in the past one million years. The paper estimates that humans (sadly) managed to achieve in five centuries what would’ve taken nature 18,000 years.

While every species that goes extinct is an irreparable loss, losing entire genera can wreak havoc on ecosystems. Sticking with the tree analogy, a limb can lose a few twigs and still provide vital sustenance for the tree as a whole. But when entire limbs (or genera) are lost, holes start to form in the canopy, and other surviving species can’t fill those ecological gaps.

“As scientists, we have to be careful not to be alarmist,” Ceballos said in a statement. “We would be unethical not to explain the magnitude of the problem, since we and other scientists are alarmed.”

Although it’s vital to preserve threatened species wherever they’re found, Ceballos and Ehrlich recommend increased efforts to preserve tropical rainforests. The World Wildlife Foundation estimates that 80 percent of known species reside in these areas, so saving these environments would have the biggest impact on preserving precious ecosystems.

Climate change is an existential threat to humanity’s continued existence on Earth, but for the many species that share this planet with us, the situation is even more dire.