Close to half of all living species on the Earth could disappear by the end of this century, and humans will be the cause. This is the Sixth Mass Extinction — a loss of life that could rival the die-out that caused the dinosaurs to disappear 65 millions years ago after an asteroid hit the planet. This time, though, we’re the asteroid. At least that’s how Elizabeth Kolbert, the author of “The Sixth Extinction,” sees it. “We are deciding,” Kolbert writes, “without quite meaning to, which evolutionary pathways will remain open and which will forever be closed.

No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy.” In “The Sixth Extinction,” Kolbert traces our understanding of extinction from the first time it was proposed as a theory in the 1740s until now, with scientists mostly agreeing that humans may be causing it. It used to be that when researchers came across old animal bones, their first goal was to identify them with a species that already existed. In 1739, for example, when a group of researchers unearthed the first Mastodon bones, they assumed they were looking at the remains of two different animals — an elephant and a hippopotamus. It wasn’t until French naturalist Georges Cuvier suggested that the bones were from “a world previous to ours” that researchers first started to consider the idea that an entire species could have existed and then disappeared. MORE