|Frequently used image depicting evolution|
For example, if we trace species A back through time we see it eventually connects at a spot I've labeled X. The same is true for species B and C: we can trace their lines back to species X.
|Example of a phylogenetic tree|
Species A, B, and C are all descendants of species X, which no longer exists. Species X is the last common ancestor species A, B, and C shared. A common ancestor is simply an ancestor that two or more species have in common, and the last common ancestor is the most recent common ancestor of two or more species. If we keep traveling down the line of A (and back in time), we see that species Z is also a common ancestor of A and B, but the last common ancestor of A and B is species X. There's a slight difference in meaning between common ancestor and last common ancestor. All of the species on this tree originally evolved from species Z, but this was a very long time ago. Usually, trees represent millions of years. Over time, species change. Species Z was one species at the start of this tree, but something happened (either gradually or suddenly) to make this species split into two different species, X and Y. Maybe one population was separated by another due to an earthquake, as time progressed, those two populations became so different from each other, they formed new species (X and Y). Species X may have been separated from a critical food source, and thus gradually began foraging only at dusk and dawn for insects, whereas the other population started interbreeding with another population made up of only white-colored individuals. Over time, the two become distinct populations and eventually distinct species.
|Close-up of some of the phylogenetic tree|
If you follow the lines, you can see that species F eventually splits again into separate species, making species F an ancestral species. It is the last common ancestor to H and I, both of which still survive to this day. Species H is more closely related to species I than it is to species C, which it last shared a common ancestor with a long time ago. That means that H and I will share more characteristics than species I and C.
|Tree showing relationship of great apes and humans|
It's important to remember that while it looks like species H and I separated from each other at a very distinct point according to this tree, that's not always the case. Populations are always evolving, and it may take a few generations or a hundred generations for a species to gradually become a separate one. It is humans who define when exactly a species arises or diverges, but the reality is that it is a process, and the exact time when one species became two is debatable. There usually isn't a right answer. Scientists are forever learning about species, finding new ones, and reassembling these trees using the best information available. The end of the tree, in our case A, B, and so forth, don't need to represent species. You could do a phylogenetic tree and go as far as genera if you'd like or go all the way to subspecies even. It depends on what your purpose is.
True or false: humans are more closely related to orangutans than gorillas.
True or false: humans are more closely related to chimpanzees than bonobos.
Answer: false. If you switch bonobos and chimpanzees in the tree above, you'd be within your right to do so. The same amount of time has passed since we split from the last common ancestor we shared with both species.
Reading trees: a quick review
Chimpanzees and humans may have split much earlier than thought
Phylogenetic tree of primates
Large phylogenetic tree based on genetics