The are several categories relating to the Tree of Life which I consider important.
The first category includes all extant creatures. By adolescence, we are familiar with thousands of animals. Scientists estimate there are somewhere between 5 and 100 million species altogether. Most are probably insects and arachnids, including over a million species of both mite and beetle.
The second category includes all species that have ever lived. This number is somewhere between 10 and 100 times greater than the number of extant creatures, therefore somewhere between 50 million and 10 billion. To me, making sense of the first category requires understanding the second. I am fascinated by the second category because most people don’t know too much about it, and it’s like visiting an alien world — there are so many unusual and fascinating creatures in the fossil record.
The third category includes all species that could ever theoretically exist. We can really blow this up to huge proportions, including species based on something besides DNA, including non-carbon-based life forms, if they are physically possible, which …