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NOTE: Someone else asked this same question a few years back, but it was incorrectly marked as a duplicate and so the question may not have been answered satisfactorily. I'm asking it again, in hopes of getting a stronger answer (if there is one). Please do not mark as a duplicate; I really appreciate it!

(Original question: How many species have existed on earth?, incorrectly marked as a duplicate of: How many organisms have ever lived on Earth?)

QUESTION: How many species have ever existed on Earth, throughout the entirety of Earth's history? For clarity, I'm not asking about the number of individual organisms (answered in 2nd link above), nor about the number of living species estimated to exist today (also cited in an answer in the 2nd link above). All species, ever (never answered in any link, to my knowledge).

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  • $\begingroup$ Species are not countable in time. $\endgroup$
    – Bryan Krause ♦
    Commented Jul 18, 2019 at 15:01
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    $\begingroup$ Not an answerable question. The problem, besides the obvious one of lack of data (no one really knows how many species exist today, let alone in the distant past), is that the idea of species is a human construct, For instance, are wolves & coyotes different species? They have different names, but have interbred to produce the coywolf. Or all the fossils of pre-humans that keep getting dug up: are they all really different species? Then when you get into microorganisms and things like horizontal gene transfer, the species line gets pretty blurry... $\endgroup$
    – jamesqf
    Commented Jul 18, 2019 at 18:07
  • $\begingroup$ I was only wondering if there was a ballpark figure, out of curiosity. I know there is no perfect way to answer the question, given the subjectivity of what constitutes a "species", and the incompleteness of the fossil record (and impossibility of many types of organisms being fossilized to begin with). I am not that strong in this particular branch of biology, so I posed the question. If there's no answer, there's no answer, and that's fine (once again, I wasn't sure since the previous asking of the question was shot down). $\endgroup$
    – user22038
    Commented Jul 18, 2019 at 20:10

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From Raup (1986).

Up to 4 billion species of plants and animals are estimated to have lived at some tlme m the geologic past (2), most of these in the last 600 million years (Phanerozoic time). Yet there are only a few million species living today. Thus, extinction of species has been almost as common as origination.

I am not sure how exactly has it been estimated. Read the paper for more details.

In 1991, Raup comments:

Countless species of plants and animals have existed in the history of life on Earth. Estimates of the total progeny of evolution range from 5 to 50 billion species. Yet, only an estimated 5 to 50 million species are alive today – a rather poor survival record. With, at the most, only one in every thousand species surviving, what happened to the others?

Mora et al. (2011) predict ~8.7 million extant eukaryotic species. So I would guess that there are >5 billion (extinct + existing) species on earth.

I guess there will be many more prokaryotes. We don't know anything about ancient prokaryotic species because there are no fossils records. I would say that it would be nearly impossible to estimate the number of prokaryotic species: even now we do not have a very reliable method of typing microbial species (people mostly rely on 16S rRNA sequencing). See What defines a microbial species?

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