Question

How many species, total, have ever lived on the Earth?

In short, we don't have an exact number. Even the estimate of the total species that live on Earth today is not well known. It may range between 10 million and 100 million and only includes plants and animals. Therefore, a calculation of all the multicellular species that have ever lived requires several important assumptions:

(1) The first animal/plant species, indicated by the fossil record of abundant multicellular life, began about 600 million years ago.

(2) The number of estimated species today is between 10 million and 100 million.

(3) The number of species had a linear increase in diversity from 1 species 600 million years ago to a range of 10 million to 100 million species today.

(4) From what we know of invertebrates, the average species lives for 4 million years. Using this logic, the result is a number that ranges between 750 million and 7.5 billion species! Factoring in bacteria and archaea (a special class of single-celled organisms) is difficult because they have such a poor fossil record. Thus, we cannot get a reasonable estimate of average species duration or diversity for anytime on Earth, making it nearly impossible to conclude anything using this approach. Nevertheless, since the existence of single-celled organisms ranges back to at least 3.5 billion years ago, it seems reasonable that the number of species that have ever lived on Earth could easily climb to more than 10 billion in total!
October 29, 2001