Friday, February 13, 2015

No, humans did not evolve from monkeys

Frequently used image depicting evolution
I often hear, "humans evolved from monkeys" or substitute monkeys for apes or chimps. Unfortunately, this is a common misconception. It's simply inaccurate and a result of poor understanding of how evolution works. Humans did not in any circumstances evolve from monkeys or chimps or lemurs. For the sake of clarity, let me repeat, humans did not evolve from monkeys or chimps or lemurs. I'm not about to go on a creationist rant but I am about to explain hopefully dispel some common misconceptions about evolution. To understand why saying humans evolved from monkeys is so wrong, we first need to take a closer look at how evolution work and we're going to start with phylogenetic trees. A phylogenetic tree is a visual representation of the evolutionary relationships between species. They're often just called trees for short and they're very useful. If we look at the one below, we see multiple species (named "A" and "B" and so forth). The nodes or areas where multiple lines come together represent common ancestors.

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
Let's take a closer look at another side of the tree and pretend that these alphabetically named species are species of monkeys. D, E, and F separated or diverged from species Y. If I look the tree over from the top to the bottom, I see that species D, E, and F are relatively young (their lines separating them from Y are relatively short), and that species Y spent a great deal of time successfully as species Y before it split into multiple species. Let's say one of the main reasons why species Y split into multiple species was because a new predator was introduced into the habitat. One population of Y changes its behavior and activity patterns to become nocturnal, thus avoiding the diurnal predator entirely. That population becomes E. Enough time passes and E can no longer breed with close relatives, D or F. Another population E finds success in foraging lower in the canopy, thus avoiding the predator, which let's say is some sort of predatory bird. E forages lower in the canopy but has to make behavioral changes to do so, feeding on different items and relying on its tail less to balance on thicker tree branches. Over time, E becomes a distinct species and cannot breed with F or D. It retains some success for a while but eventually it begins to lose numbers due to feeding competition from other animals occupying this part of the canopy, and E goes extinct. This is why species E's line does not extend all the way to the top of the tree, or the present time. It did not survive. The same is true for species D, which managed to adopt certain vigilance behaviors to avoid the predator, but was sadly wiped out from a disease that destroyed the population.

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
Now, all you have to do is apply this to primates. Chimpanzees, lemurs, and so forth are all living species. Replace A with lemurs and I with humans and you can see immediately that we did not evolve from lemurs. A does not come from I. That's obviously wrong. However, A and I do share a common ancestor, Z. Now, these A and I have been evolving, over time, separately for a while. They've taken different paths, but they do share a common ancestor, a common ancestor, Z, which is no longer alive. It may seem like a small difference, but the number of people who actually believe that we did evolve from monkeys is perhaps greater than we'd all like to believe. By saying, we shared a common ancestor with monkeys or we evolved from an ape-like ancestor as did chimpanzees, some of the confusion might be avoided. Or someone may ask you to clarify, and you can explain that humans did not, in fact, evolve from chimpanzees. Looking back to the first image depicting evolution of humans, if you said that first animal was a gorilla, that would be inaccurate. If you said it was an ancestor of both gorillas and humans, you'd be okay.

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.
Answer: false
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.

Further reading:
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

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