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.)

Advances in molecular biology led to the use of molecular methods to identify patterns of genetic variation at the DNA level. This enabled a more robust assessment of the patterns of genetic variation at the individual level that contribute to the study of the genetic affinities of human populations around the world. This paper focuses on how genetic variation among sub-Saharan African populations has contributed to advancing our understanding of population history in Africa and human evolution and discusses ethical issues related to conducting research on human subjects. Using mitochondrial DNA (in both women and men) and Y-chromosome DNA (men only), it is possible to trace an individual’s maternal and paternal lineage ancestry, respectively. Public curiosity about its ancestry sparked interest in genetic ancestry testing services and population genetic research, thus contributing to public participation in science.

Lifespan is just one component of a species life history. To understand human longevity from an evolutionary perspective, it is important to consider the human species’ phylogenetic history and the evolution of the entire human life cycle. This chapter extends previous fundamental reviews in the light of recent findings, and with particular emphasis on the evolution of longevity of the human species. It first compares the primate life cycle to that of other terrestrial mammals, and highlights the evolution of the slow pace of life observed in primates. It then compares the life cycles of humans and other primates, emphasizing the peculiarities of the human life cycle. The chapter outlines the main theories explaining the evolution of these peculiar life history traits that occurred since the human-chimpanzee divergence, linking these to the evolution of human reproduction, ontogenesis, diet and cognition. It then emphasizes the pivotal roles of sociality and intergenerational transfers for understanding the joint evolution of the human life cycle, biology and cognitive, linguistic and social capabilities. Together, this finally allows a contemplation of the most probable scenario joint evolution of human reproduction, ageing and longevity.

This chapter reviews what can be gleaned about human sexuality from the evolutionary and ethnographic record. Ancestral human sexuality leaves neither fossil nor archaeological evidence, but inferences about how humans mated, consorted, parented, formed partnerships, and aggregated into families can be drawn from two large and growing bodies of work, both discussed in this chapter. The first are anatomical and biological indicators of ancestral mating patterns inferred from fossil evidence as well as observations from nonhuman primates. The second is ethnographic research across an array of contemporary human societies, which highlights variation in mating, marriage, and family structure. Together, biological indicators and cross-cultural patterns shed light on the legacy, constraints, and possibilities carried forward into the diverse and variable expression of human sexuality today. Humans have a deep ancestry in a social structure of males and females living in social groups together, although how humans organize themselves is structurally different from anything observed in our closest relatives. Not only do families form around long-term pairbonds in all societies, but there is also a great deal of flexibility in who constitutes the pairbond, the families that surround them, and in the prevalence of extra-pair relationships.

Among the diversity of perspectives for studying the nexus of evolution and human behavior, human behavioral ecology (HBE) emerged as the study of the adaptive nature of behavior as a function of socioecological context. This volume explores the history and diversification of HBE, a field which has grown considerably in the decades since its emergence in the 1970s. At its core, the principles of HBE have remained a clear and cogent way to derive predictions about the adaptive function of behavior, even as the questions and methods of the discipline have evolved to be more interdisciplinary and more synergistic with other fields in the evolutionary social sciences. This introductory chapter covers core concepts, including methodological individualism, conditional strategies, and optimization. The chapter then provides an overview of the state of the field, including a summary of current research topics, areas, and methods. The chapter concludes by emphasizing the integral role that human behavioral ecology continues to play in deepening scholarly understandings of human behavior.

Darwin’s Origin of Species [GK26][GK27](1859), despite its almost complete silence about human evolution, was the catalyst for widespread discussion and debate during the 1860s about the history and future of humanity; about slavery and the identity of the human ‘races’; and about competition and struggle in Victorian society. Three popular novels of the early 1860s – George Meredith’s Evan Harrington [GK28](1860), Charles Dickens’s Great Expectations [GK29](1860–61), and Charles Kingsley’s The Water-Babies [GK30](1862–63) – illustrate how quickly Darwin’s ideas were appropriated into fictions dealing with race and social class in the decade of the American Civil War, the Morant Bay Rebellion, and the Second Reform Bill. Although generally associated with literary realism, Darwin’s work may be better aligned with the new narrative form so popular in the 1860s: sensation fiction.

The chapter investigates the evolutionary origins of human moral cognition. It discusses the relation of moral and legal theory and evolutionary theory. It reconstructs different perspectives on the evolutionary process. Influential current approaches are evolutionary psychology, theories of ontogenetic development on the basis of joint intentions and what one may call "evolutionary pluralism." The latter approach underlines the importance of many factors beyond natural selection that determine the genesis of a certain species and the stochastic nature of evolution. The methodological and theoretical foundations of these competing approaches are discussed. The problem of finding evidence for the properties of early humans is investigated. That state of research in paleoanthropology is recapitulated and discussed in terms of what it teaches us (and leaves undecided) about the genesis of human cognition, in particular moral condition. Taking into account the evidence and the most plausible evolutionary theory, it is argued that there is no compelling reason to assume that the moral principles underlying human rights are irreconcilable with human psychology.