1/27/2020 Hominin Taxonomy and Phylogeny: What's In A Name? | Learn Science at Scitable This page has been archived and is no longer updated price drop price drop price drop price drop price drop Clarks India HUMAN FOSSIL RECORD | Lead Editor: Holly Dunsworth Hominin Taxonomy and Phylogeny: What's In A Name? By: Kieran P. McNulty (Evolutionary Anthropology Lab, University of Minnesota) © 2016 Nature Education Citation: McNulty, K. P. (2016) Hominin Taxonomy and Phylogeny: What's In A Name? Nature Education Knowledge 7(1):2 At the most basic level, human evolution is articulated through classifications of, and evolutionary relationships among, hominin species. This article presents a basic taxonomy and phylogeny of hominins, but also further explores the factors that confound systematics in human evolution. Aa Aa Aa The practice of biological classification appeared early in the evolution of life: Is this safe to eat/not safe to eat? Is that a predator/not a predator? Are these potential mates/not potential mates? It is in our nature, then, to classify our surroundings. Within the context of biology this practice takes on special importance, as scientists from many biological disciplines work toward a single classification system that incorporates all organisms that have ever lived. Both the practice and the product of this grand biological classification are called taxonomy. The most widely used taxonomic system was formalized by Carolus Linnaeus and comprises a simple nested hierarchy wherein similar organisms group together at one taxonomic rank, and those groups cluster into successively broader groups at higher ranks. Figure 1 shows a taxonomy of human ancestors and their extant African ape relatives. This nested hierarchy allows different degrees of similarity to be represented at different ranks. Note, however, that Linnaean taxonomy predates modern evolutionary theory; whereas Linnaeus organized living organisms according to different levels of similarity, it was Darwin and others who explained these differing degrees of similarity through common ancestry. The more closely related two groups are, the more similarities they are likely to share. The series of evolutionary relationships among a group of organisms is termed phylogeny. https://www.nature.com/scitable/knowledge/library/hominin-taxonomy-and-phylogeny-what-s-in-142102877/ 1/8 1/27/2020 Hominin Taxonomy and Phylogeny: What's In A Name? | Learn Science at Scitable Figure 1: Taxonomy for human ancestors. Linnaean ranks are listed across the top, with vertical lines to indicate the indentation of corresponding taxon names; ranks and taxa are also correlated by color. Ranks above the genus level include the proper taxon name (always starting with a capital letter) as well as the common name in parentheses. Hence, to refer to the group of African apes and humans, one could say either the Homininae or the hominines. The genus and species names (commonly called the "scientific name") are always written in italics, and the genus starts with a capital letter whereas the species is always lowercase. Note that the species name is never written alone, and thus each species is preceded by an abbreviation for its genus. Although nearly all workers recognize the close relationship between Homo and Pan, to the exclusion of Gorilla, there is not yet a broadly used taxonomic rank to demarcate this association. Courtesy of Kieran P. McNulty The Relationship Between Taxonomy and Phylogeny It is commonly understood that taxonomy should reflect phylogeny - organisms should be grouped according to their evolutionary history, their relatedness. This makes intuitive sense because common ancestry is the only factor that unites four billion years of life on Earth. This connection between taxonomy and phylogeny means that new discoveries or other data that change our understanding of evolutionary history often result in the shuffling of taxa and of taxonomic names. An excellent example of this shuffling is the change in usage of the term "hominid." Traditionally, only human ancestors were placed in the family Hominidae (and thereby referred to as hominids). This reflected a view that humans are substantially different from the great apes, which were placed in the Family Pongidae (pongids) (Figure 2a). However, overwhelming genetic evidence has since demonstrated that humans, chimpanzees, and gorillas are much more closely related to each other than to the orangutan (e.g., Sarich, 1971; Caccone & Powell, 1989; Ruvolo, 1994). Thus, there is no genetic support for grouping the great apes together in a distinct group from humans. For this reason, many researchers now place all species of great ape and human within a single family, Hominidae - making them all proper "hominids" (Figure 2b). https://www.nature.com/scitable/knowledge/library/hominin-taxonomy-and-phylogeny-what-s-in-142102877/ 2/8 1/27/2020 Hominin Taxonomy and Phylogeny: What's In A Name? | Learn Science at Scitable Figure 2: Changing phylogeny of the apes and implications for taxonomy. (a) A traditional phylogeny of modern apes, where orangutans (Pongo), gorillas (Gorilla), and chimpanzees (Pan) were thought to be very closely related. In this scheme the lesser apes, gibbons and siamangs (Hylobates), are put into their own family of hylobatids; great apes are grouped together in the Pongidae (pongids), and only the human lineage was included in the Family Hominidae, the hominids. (b) This is a modern understanding of ape phylogeny, where the great apes no longer represent a distinct group from humans. Here, there is no support for a separate Family Pongidae and therefore great apes and their ancestors are grouped within the Family Hominidae. For this reason, the term "hominid" is now typically applied to all great ape and human species not merely the lineage of humans. Courtesy of Kieran P. McNulty Such taxonomic changes have downstream effects as well: African apes and humans are now distinguished from orangutans at the subfamily rank Homininae (hominines), and the human lineage is separated at an even lower rank of Tribe Hominini. Thus, the common term for the lineage of fossil and modern humans is "hominin." Furthermore, the traditional term "australopithecine," which grouped Australopithecus and Paranthropus in the subfamily Australopithecinae, becomes invalid under this revised taxonomy; in the context of Linnaean taxonomy, one cannot nestle a higher- ranked subfamily (Australopithecine) within a lower-ranked tribe (Hominini) (see Figure 1). On one hand, this change in terminology over the years illustrates the proper advancement of science, whereby new evidence about evolutionary history helps to refine our taxonomic models. However, the necessary linguistic shift has been slow to gain acceptance among some researchers. As well, the popularity of terms like “hominid” and “australopithecine,” which seeped into public awareness in the 1960s-70s, means these terms will likely persist in their traditional meanings for communicating with the public. Hominin Taxonomy and Phylogeny One model of hominin evolution is illustrated by the phylogeny in Figure 3, presented here as a vehicle for discussion rather than a robust hypothesis. An obvious point of contention is the status of the earliest hominins, known from only small isolated samples; relationships both among these species and to the later hominins are not yet resolved. And, while other branches of the human evolutionary tree are vigorously debated in the literature, the usual signposts of phylogenetic uncertainty (e.g., dashed lines, alternate branches, question marks) have been omitted from Figure 3 for simplicity. Instead, a few of these issues are highlighted below, specifically in the context of taxonomic and phylogenic obstacles: ancestry, variation, and biological complexity. Figure 3: A phylogeny of hominin species. Approximate geological age ranges are included for each. Black lines indicate a phylogenetic connection, though these relationships are proposed primarily for discussion and are not well-tested hypotheses. Several species are left unconnected https://www.nature.com/scitable/knowledge/library/hominin-taxonomy-and-phylogeny-what-s-in-142102877/ 3/8 1/27/2020 Hominin Taxonomy and Phylogeny: What's In A Name? | Learn Science at Scitable due to the many possible ways in which they might related to the other species. The box highlights a section of the phylogeny that is used in Figure 3. Courtesy of Kieran P. McNulty Australopithecus and the problem of ancestry Five species of Australopithecus are recognized here, though new discoveries are likely to add to this number. There is good evidence that A. anamensis and A. afarensis represent evolution within a broad lineage (Kimbel et al., 2006), but other relationships in the genus are not well understood. It may be that the genus is an amalgamation of stem species that are not closely related (see below), though they seem to share a common adaptive grade. Most researchers would agree that both Paranthropus and Homo evolved from some Australopithecus-like ancestor(s), though which is still contentious. This idea of ancestry, however, presents difficulties for our efforts to match taxonomy to phylogeny. For example, if the phylogeny in Figure 3 were correct, then A. africanus and its descendants would be more closely related to the genus Homo than other species of Australopithecus; likewise, A. garhi would be more closely related to Paranthropus. How, then, can our taxonomy be made to reflect these evolutionary