Last week I talked about the evolutionary origins of birds, which are dinosaurs in every sense. This week I will talk about our own species and where we came from. Spoiler alert: no, we are not dinosaurs.
Not dinosaurs, but we are apes. Actually, let me back up a little bit. Humans are animals. People constantly talk about humans AND animals, as though there is some sort of difference between the two. In biology, an animal is defined as anything with the following traits: multicellularity (check), no cell walls (check), sexually reproducing (check). We have all of the traits of animals, and our evolutionary history is in the animal lineage. Anyway, back to apes.
Apes are a particular lineage of primates that includes the gibbons, siamangs, gorillas, orangutans, chimpanzees, and humans. The “great apes” — a group to which we also belong — includes just the gorillas, orangutans, chimpanzees, and humans. (The gibbons and siamangs are pretty good, but they aren’t great.) Back in the day, it was easy for people to believe that we weren’t. Scientists knew that we were closely related to the apes, but didn’t believe that we were actually in the ape lineage. The anthropologists studying humans and our relatives believed that, of all of the apes, humans were the ”out-group.” Said differently, they believed that the following cladogram was correct:
Incorrect ape cladogram. Humans are most closely related to chimpanzees.
Humans (and our recent ancestors and relatives, like the neanderthals, Homo erectus, Australopithecus afarensis, etc.) were classified in the group “Hominidae” and the other apes were in the group “Pongidae.” But then they did more research and found that this was not correct. With the advent of genetic testing, it was discovered that the human genome is about 99% identical to that of the chimpanzee. Human anatomy and physiology is nearly identical to that of the chimpanzees. Humans have 23 pairs of chromosomes, compared to 24 pairs in the other great apes. Upon closer examination, our chromosome #2 is the result of the end-to-end connection of two of the chromosomes from the other great apes.
Correct ape cladogram
From this, it is clear that we cannot continue to call humans hominidae and the non-human apes “Pongidae” for the same reason that we have to call birds dinosaurs.
As it stands, there are two species of chimpanzee: the common chimpanzee (Pan troglodytes) and the bonobo or “pygmy chimpanzee” (Pan paniscus). We are so similar to these two chimpanzees that some biologists advocate classifying humans (Homo sapiens) and chimpanzees in the same genus — either reclassifying humans as Pan sapiens or the reclassifying the chimpanzees as Homo troglodytes and Homo paniscus.
In addition to being apes, we are also fish. This usually comes as more of a shock to people than the news that we are apes. It is not through a technicality or semantics that we are fish. We are fish in the same way that we are apes or that birds are dinosaurs. We may not look like fish outwardly, but we are made up of traits — very important traits — that first evolved in organisms that were unambiguously fish: Before fish, there were no animals that had a bony skeleton. Only fish and their descendants have this trait. This alone places us in the group “Osteichtheys” which literally means “bony fish.” Early vertebrates had no jaw. The jaw first evolved from a “gill arch,” which is a bony structure that supports the gills in fish. Early vertebrates had no teeth, either, until they evolved out of fish scales. Human embryos have gills, one of which develops into a jaw. In rare cases, babies are born with (non-functioning) gills.
All vertebrates, including humans, have gill slits as embryos. Image from evolution.berkeley.edu
Most of the fish that we know are not the type of fish that we evolved from (and still are). There are three main groups of fish: The first are the cartilaginous fish (Chondrichtheys), which include the sharks and rays. The second are the ray-finned fish (Actinopterygii), which include most of the fish that we eat: trout, tuna, cod, halibut, flounder, bluefish, etc. The third are the lobe-finned fish (Sarcopterygii), which include the coelocanths, the lungfish, the amphibians, the reptiles, and the mammals. The lobe-finned fish have fleshy, limb-like fins that were the anatomical basis for the arms and legs of the land vertebrates.
This cladogram shows that humans evolved from fish.
We are fish. We are fish that crawled out of the water and evolved for life on land, but we are still fish. Outwardly, we do not look much like fish, but the anatomical, developmental and genetic traits that we have give away our heritage. That we are fish or that we are apes does not in any way detract from our humanity. Our evolutionary heritage connects us to other organisms in a way that enriches our understanding of ourselves.
Have a topic that you want me to cover? Let me know in the comments section.
Follow me on twitter @CGEppig
ChristopherEppig 
October 27th, 2014 at 9:02 pm
[…] and all of the rest of life. It is all of the rocks and minerals and water and air. Even humans, which are animals, are part of nature. Everything beyond our planet is part of the natural world, as well. All of the […]