The hominin fossil record: taxa, grades and clades

Abstract

This paper begins by reviewing the fossil evidence for human evolution. It presents summaries of each of the taxa recognized in a relatively speciose hominin taxonomy. These taxa are grouped in grades, namely possible and probable hominins, archaic hominins, megadont archaic hominins, transitional hominins, pre-modern Homo and anatomically modern Homo. The second part of this contribution considers some of the controversies that surround hominin taxonomy and systematics. The first is the vexed question of how you tell an early hominin from an early panin, or from taxa belonging to an extinct clade closely related to the Pan-Homo clade. Secondly, we consider how many species should be recognized within the hominin fossil record, and review the philosophies and methods used to identify taxa within the hominin fossil record. Thirdly, we examine how relationships within the hominin clade are investigated, including descriptions of the methods used to break down an integrated structure into tractable analytical units, and then how cladograms are generated and compared. We then review the internal structure of the hominin clade, including the problem of how many subclades should be recognized within the hominin clade, and we examine the reliability of hominin cladistic hypotheses. The last part of the paper reviews the concepts of a genus, including the criteria that should be used for recognizing genera within the hominin clade.

Fig. 1

Taxa recognized in a typical speciose hominin taxonomy. Note that the height of the columns reflects current ideas about the earliest (called the first appearance datum, or FAD) and the most recent (called the last appearance datum, or LAD) fossil evidence of any particular hominin taxon. However, the time between the FAD and the LAD is likely to be represent the minimum time span of a taxon, as it is highly unlikely that the fossil record of a taxon, and particularly the relatively sparse fossil records of early hominin taxa, include the earliest and most recent fossil evidence of a taxon.

Fig. 2

Options for an 8–5 Ma African higher primate taxon that is more closely related to Homo and Pan than to Gorilla. Scheme A assumes no homoplasy and the only options within it for such a taxon is that it is the last common ancestor (LCA) of modern humans and chimps/bonobos, a primitive hominin, or a primitive panin. Scheme B takes into account the probability of homoplasy, and in addition to the above options such a taxon could be a member of an extinct clade that is the sister taxon of the LCA of modern humans and chimps/bonobos, or a hitherto unkown clade that is the sister taxon of the earliest hominins or panins.

Fig. 3

Hypothesis of the cladistic relationships among fossil hominins and modern humans. This cladogram was determined to be the most parsimonious one in a recent cladistic analysis of fossil hominins (adapted from fig. 10 in Strait & Grine, 2004).