When I am on Twitter, every now and then a witty or funny tweet catches my attention. I laugh, and sometimes re-tweet. More often, however, I read tweets that cause anxiety and make me frown at my computer screen with the countenance of a distraught fish. I am talking about tweets like this one (Fig. 1):

Here is what Mr. Allen most likely imagines when he thinks about evolution: At some point in the past, this monkey-like creature with long limbs that you can see at the zoo – what Mr. Allen calls an “ape” – had a baby that looked less “ape-ish” and more “human-ish”. Over many generations, this process culminated in us (Fig. 2). The small-scale equivalent (if you “zoomed in”) would be the linear genealogical chain from grandparent to grandchild.

Mr. Allen’s question could be a publicity stunt, or maybe he is just a provocateur, but many of the 50,000 “likes” his tweet has at the moment are probably genuine. This gives me cold sweats. Why? Let me use the smaller scale generational parallel to rephrase his tweet, even if I risk jumping the gun on my own argument: “If I am grandfather Allen’s grandson, how come I have Allen cousins?”. This question reveals a fundamental misunderstanding of the natural phenomenon of evolution. For an evolutionary biologist living in the 21st century like me, this is as misguided as asking why people on the other side of the world do not fall into the void of space.

Evolution is not a linear process that starts with more “primitive” looking organisms we can observe today, and ends in mankind (as shown in Fig. 2). Erase this simplistic cartoon from your mind, Mr. Allen. Instead, look intently at figure 3. Biologists have given diagrams like this a fancy name: cladogram. Unlike figure 2, a cladogram captures the most important (and ongoing!) aspect of the evolutionary process: “branching,” or what biologists refer to as cladogenesis. Cladogenetic events are the moments in time during which one species “splits” into two species – these events are also known as speciation events. In figure 3, these events are represented by the points at which one line “bifurcates” into two lines.

The branching nature of cladogenesis has two important consequences. First, because two or more new species always originate from an ancestor species (and this process has been occurring since the origin of life), any two species we observe in the present are related. The truth might hurt, but yes, humans and chimpanzees are (distant) relatives. And so are blue whales, white sharks, sequoia trees, mushrooms, flies, earthworms, bacteria, etc. They are all relatives of yours.

This idea of universal relatedness, also known as common descent, was proposed by none other than Charles Darwin himself in On the origin of species [1], but also by the often neglected Alfred Russell Wallace [2]. Common descent is arguably the most important, overwhelmingly accepted idea in biology.

The second consequence is that when biological classification is performed, cladogenesis leads to a natural hierarchy of groups, in which one can be nested into another. Sometimes one look is worth a thousand words, so take a peek at figure 4. These primate species have many characteristics that allow us to classify them into successively more inclusive groups. Hominidae consists of all humans, chimpanzees and bonobos, gorillas, and orangutans. If we add gibbons to the mix, we now have the Hominoidea (or “apes”). Finally, if we include old world (e.g., a macaque) and new world (e.g., a marmoset) monkeys, we get the Anthropoidea (or “simians”). Hominids are nested inside the hominoids, which in turn are nested inside the anthropoids. And the more deeply nested a group is, the more alike its species will tend to be.

For the most part, today’s biological classification observes the rules of cladistics, a framework for studying biodiversity proposed by German entomologist Willi Hennig [3]. Cladistics is a big subject, but we can focus on its main tenet: The only biological classifications that make evolutionary sense are those nested groups (such as those highlighted in Fig. 4) which include an ancestor and all its descendants. These nested groups are called clades.

Think about clades as being incredibly large and old “families” that include a great-great-great-great-(many, many blog pages after)-great-grandfather/mother and all its gazillion descendants into the present. The “apes” (also known as Hominoidea, as defined above), are a clade to which we humans belong, together with bonobos, chimpanzees, gorillas, orangutans, and gibbons. We are apes ourselves.

We now have everything we need to answer Mr. Allen’s question. If you have just skimmed the rest of the post, here is the take-home message.

We did not evolve from a modern, living ape, like a chimpanzee. We evolved and descended from the common ancestor of apes, which lived and died in the distant past. This means that we are related to other apes and that we are apes ourselves. And alongside us, the other living ape species have also evolved from that same common ancestor, and exist today in the wild and zoos.

Being able to observe ape species other than us humans in the present moment poses no problem to evolution whatsoever — if anything, observing and learning about them can teach us more about ourselves!

References
[1] Darwin, C. R. (1859). On the origin of species by means of natural selection, or preservation of favoured races in the struggle for life. London: John Murray.
[2] Darwin, C. R., Wallace, A. R. (1858). On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection, Zoological Journal of the Linnean Society, 3(9), 45–62.
[3] Hennig, Willi (1966). Phylogenetic Systematics. University of Illinois Press.

Edited by Lana Ruck and Liz Rosdeitcher.