Extinction has been a natural part of the planet's evolutionary history. In fact, 99% of the four billion species that have evolved on Earth are now gone.

And the process continues.

The International Union for Conservation of Nature (IUCN) Red List has tried to estimate the number of extinctions over the last five centuries but unfortunately, we don't know everything about all of the world's species over this period, so it's likely that some will have disappeared without us even knowing they existed in the first place, so its numbers can be an underestimate.

Still, that's the best guess we have. The IUCN Red List estimates that 900 species have gone extinct since 1500.

To understand the biodiversity problem we need to know how many species are under pressure; where they are; and what threats are they facing.

To do this, the IUCN Red List of Threatened Species evaluates species across the world for their level of extinction risk. It does this evaluation every year and continues to expand its coverage.

But the IUCN has not evaluated all of the world's known species. In many taxonomic groups, it has assessed only a very small percentage. In 2021, for example, it analyzed only 7% of described species.

As we'd expect, animals such as birds, mammals, and amphibians have seen a much larger share of their species assessed – more than 80%. On the other hand, only 1% of insects have. And less than 1% of the world's fungi.

The lack of complete coverage of the world’s species highlights two important points we need to remember when interpreting the IUCN Red List data:

1. Changes in the number of threatened species over time do not necessarily reflect increasing extinction risks;

2. The number of threatened species is an underestimate.

In total, the IUCN Red List has evaluated 40,084 species across all taxonomic groups to be threatened with extinction in 2021.

But as noted before, since birds, mammals, and amphibians are the most well-studied groups, their numbers are the most accurate reflection of the true number (and therefore, understudied groups such as insects, plants, and fungi will be a large underestimate).

Many think that humans are also doomed to disappear. Paleontologist Henry Gee thinks the most insidious threat to humankind is the so-called extinction debt.

"There comes a time in the progress of any species, even ones that seem to be thriving, when extinction will be inevitable, no matter what they might do to avert it," Gee wrote in Scientific American.

"The cause of extinction is usually a delayed reaction to habitat loss. The species most at risk are those that dominate particular habitat patches at the expense of others, who tend to migrate elsewhere, and are therefore spread more thinly. Humans occupy more or less the whole planet, and with our sequestration of a large wedge of the productivity of this planetwide habitat patch, we are dominant within it. H. sapiens might therefore already be a dead species walking."