Outside of our fellow mammals, our next closest relatives are reptiles. As both birds and mammals are warm blooded (endothermic) and have four-chambered hearts, one might be tempted to think that the sister group to mammals would be birds. But the story is much more complicated than that, especially because birds are actually reptiles.

Reptiles include four main lineages: (1) turtles, (2) lizards and snakes, (3) crocodilians, and (4) dinosaurs, including birds. Indeed, birds are reptiles – birds are a surviving lineage descended from bipedal predatory dinosaurs! In decades past, there were five “classes” of vertebrates (animal groups with backbones): fishes, amphibians, mammals, reptiles, and birds. In fact, many basic treatments still list these groups. For example, Encyclopedia Britannica still has an article entitled: “Five Vertebrate Groups.” But there are major problems with two of these old groups: neither fishes nor scaly reptiles are monophyletic.

I have argued that one of the major misconceptions about evolution and the tree of life is that some species or lineages are considered more “primitive” than others – this chapter will delve more deeply into this misconception and one of its key causes. Across the tree of life, certain lineages – including the platypus, lungfishes, and mosses – are frequently labeled as more primitive than other members of their groups. Mammals provide several good case studies demonstrating the reasons for this longstanding misperception. Researchers, journalists, and filmmakers all seem obsessed with discussing certain lineages that somehow seem primitive to them. This misconception about primitive lineages is problematic for two major reasons. First, it leads to a general misunderstanding of evolution, which can lead to fundamental misunderstandings across all of biology, including human health.

Fossils provide a unique window into how evolution has unfolded. In particular, transitions in the fossil record provide compelling evidence for how major evolutionary changes have happened. One of the most well-known transitions is from fish-like vertebrates to the first land vertebrates – our earliest tetrapod ancestors. (The word tetrapod refers to the groups of vertebrates with four legs, namely mammals, reptiles, and amphibians.) Paleontologists had known that transitional fossils connecting aquatic and terrestrial vertebrates must exist. There were abundant fossils of vertebrates with fins from around 400 mya, and there were abundant fossils of terrestrial tetrapods with limbs from around 350 mya. But key fossils were missing – those that could show details of how the evolutionary crawl onto land had occurred.

If we think of ourselves as the “highest” forms of life, we often think of Bacteria as the “lowest” forms of life. We also think of Bacteria as ancient, “primitive,” and ancestral. As discussed for many other extant branches of the tree of life, these views are misleading. But these views may be especially hard to jettison when thinking of Bacteria – aren’t they more ancestral than we are? But we must always come back to this idea: Bacteria are not our ancestors – they are extant cousins. As will be detailed below, all lineages of organisms descended from the LUCA; the major lineages of life did not descend from Bacteria.

The clade Bacteria includes species that are ecologically essential (e.g., as decomposers that impact the carbon cycle) and that comprise key organisms of our microbiome (e.g., the symbiotic Bacteria normally found on our skin and in our digestive tracts). Bacteria also cause many diseases, including stomach ulcers (Helicobacter pylori), tetanus (Clostridium tetani), and acne (Cutibacterium acnes).

This chapter begins with the strong statement that fish do not exist as a true evolutionary group. Of the five traditional “classes” of vertebrates, fishes are the most problematic. The concept “fish” is wildly paraphyletic. In contrast, extant amphibians form a monophyletic clade. Mammals are also a true evolutionary group. In the previous chapter we learned that the former paraphyletic group Reptilia can be fixed by recognizing that birds are reptiles.

But there is no simple fix for fishes. One possible solution is to say that all tetrapods are fishes too. In other words, you and I and frogs and birds would all be fishes. That could work and it does reflect true evolutionary relationships, but it makes the former concept fishes fairly useless. Another solution is to recognize at least six separate lineages as distinct monophyletic groups.

For decades, biologists have assumed that our most distant animal cousins were sponges (Porifera). This seemed to make a lot of sense, because sponges are very different from us and from all other animals. Sponges do not have different types of tissues, such as skin, muscles, and nerves. Their colonies of cells form the colorful but irregular shapes that are common on coral reefs. There is no way to cut a sponge into two equal halves – adult sponges are asymmetrical. Surely animals such as this must be very distantly related to us, no? (Note that for this chapter, I have switched things up to talk about our most distant animal relatives first.)

But beginning around 2010, new data began to emerge suggesting that another group of animals, the comb jellies, might be our most distant animal relatives. Comb jellies, also known as ctenophores (Ctenophora), are aquatic organisms with generally translucent gel-filled bodies.

According to Aristotle and Linnaeus, there were only two “kingdoms” – Plantae and Animalia. In the 1800s, Haeckel carved kingdom “Protista” off of Linnaeus’ Plantae. Kingdoms for Fungi and Bacteria (Monera) were later added. By the time I was in secondary school, I learned a five-kingdom system. The five “kingdoms” that I learned are still frequently used in biology lessons: animals, plants, fungi, protists, and bacteria. But we now know that a five-kingdom story is so simplified as to be misleading, and it tells us very little about the broad tree of life. Back then, in the 1900s, our limited understanding made things seem more simple, but recent DNA sequence data indicate that the groupings are much more complex.

The five-kingdom system was first proposed in 1969. (1) Animalia were multicellular creatures that eat other organisms. (2) Fungi were generally multicellular decomposers that fed by a network of filamentous cells. (3) Plantae included especially the land plants.

Chimpanzees are not our ancestors! Rather, they are our closest living cousins. Approximately 7 mya there was a species of ape in Africa, the common ancestor that you and I share with the chimps. That species was not a chimpanzee – we know that thousands of changes in DNA have occurred in the descendant lineages since that ancestor. And many resulting skeletal and biological changes have occurred in both the human lineage and the chimpanzee lineage since that ancestor.

The idea that humans descended from chimpanzees is one of the most common misconceptions about evolution. The notion that we evolved from chimps fits well with the concept of the ladder of progress. We might think that chimpanzees are more “primitive” than we are, so if evolution were a progression toward more “advanced” forms, then we might think that the other living apes evolved first, and that we evolved from those apes. We might think that chimpanzees and gorillas are older species, and that Homo sapiens is a younger species that evolved more recently.

Imagine looking out on the plains of Africa sometime several hundred thousand years ago. You see a group of people – perhaps a family group with grandparents, parents, adolescents, and younger children. You can sense their connection to you – they are fellow humans and you recognize the key features that we all share today. Perhaps some of them are sharing meat from a gazelle they have killed. Others might be gathering fruit or seeds. The children might be running around chasing one another. Imagine a young woman in that clan, perhaps in her early twenties. She could be a woman that you and I and every other living human can trace our ancestry back to. Such a woman lived in East Africa approximately 150,000 years ago; she is a common ancestor that you and I share, along with every other human currently alive on Earth. We all inherited a key piece of our DNA from her. This is a segment of DNA that you inherited from your mother, and she from her mother, and she from her mother … all the way back to this woman who lived perhaps in present-day Kenya, Tanzania, or Ethiopia. She has been nicknamed “mitochondrial Eve.”

All species on Earth share common ancestry – we are all part of the same family tree. The tree of life is a representation of how all those species are related to one another. All living species on Earth are the product of billions of years of evolution, so all are evolutionary equals in that way. However, we tend to think of life in a hierarchical way. We think there are lower animals and higher animals. We may incorrectly think that species of bacteria are old and primitive, and that humans are recent and advanced. Many news articles about evolution can feed into the perceptions that some species are younger, more advanced, or more evolved. But all of those perceptions are misleading. Each of these present-day species are our evolutionary cousins. All species alive today are the product of the same 3.5 billion years of evolutionary change, each adapting to their own environment. (Note that species are the units of evolution, frequently defined based on the distinctiveness of their appearance and genetics, and often on their ability to interbreed and produce fertile offspring.)