The Size of the Problem
Carolus Linnaeus kept good records. In one edition of his book Systema Naturae he listed 4,235 species of animals and about 6,000 species of plants. He knew enough that he knew he did not know everything, so he estimated that there could be as many as 10,000 animal and another 10,000 plant species, but this was probably the outer limit.
One hundred years later, about Mendel's time, the lists of plant and animal species had increased beyond all these estimations, and microscopes had revealed to Louis Pasteur and others that there were very large numbers of very small creatures in humble drops of water. Estimates increased and guesses of 150,000 plant species and 7 million animal species were considered reasonable.
We are still guessing, but the numbers keep getting bigger and bigger. Today there are known to be 740,000 insect species; 290,00 plants; 23,000 algae; 30,000 protozoa; 66,000 fungi; 4,600 monera; and 280,000 other animals, and thousands more are added to this list every year.
The results were staggering. Thousands of new insects were found in this one tree and when a similar experiment was done with a different tree, a whole new range of insect species were discovered. Nature, it seemed, had a lot of species we didn't know about.|
Depending on how the estimates are made, there could be as few as 20 million species of organisms somewhere on the planet, or over 35 million. It depends on who you ask, and the number keep creeping up as single celled organisms are found miles deep in the rocky mantle of the earth and inhabiting dark ocean troughs far away from the sun light.
As the numbers for currently known or estimated species keep rising, so do the estimates for total number of species of organisms that ever lived. Constant evolution and extinction have repeatedly covered the earth in unique species that have come into existence lived for a time, and then vanished as conditions or circumstances have changed.
At the moment there is no reasonable way of accurately estimating how many species there have been on our planet since life first arose. The number of 10,000 million species is sometimes used, but all that figure gives us is some idea of the order of magnitude, not any kind of accurate number.
Which ever way you look at it, however, both qualitatively and quantitatively, the amount of life on earth is large.