I meant to write something about Wood and Harrison’s article, The evolutionary context of the first hominins when it first came out, but have been a bad, lazy blogger and am just now getting around to it.
Before going further, I need to clarify some terminological and conceptual issues. Let’s start with the notion of crown groups. Crown groups are composed of currently extant species going back to the common ancestor of that group. An example will make that clear. Take the order primates, for example, the crown group primates is composed of all currently extant forms back to their common ancestor. The concept though, is somewhat relative in that you can have crown group primates, crown group haplorhines, or crown group panin (e.g. chimps and bonobos back to their common ancestor). Crown groups are also monophyletic. In the picture below crown groups are the dark blue and orangish colored groups.
Stem groups are extinct forms more closely related to a given crown group than any other. Again, an example will make this clearer. Take the crown group composed of, well, human beings – Homo sapiens. The stem group would be anything more closely related to humans than to the crown group composed of chimps and bonobos.
Of, course, anything more closely related to chimps and bonobos than human would be a stem panin. One could take this even further and say that anything more closely related to gorillas than to humans and chimps is part of the gorilla stem group – and vice versa. At this point I should mention that the stem group plus the crown group is referred to as the total group. So, how are the traits that define crown groups distributed in stem groups? At this point, let me quote again two passages from Valintine’s On the Origin of Phyla:
Consider the changing morphospace within the stem group of a phylum when it is traced back through successively earlier nodes from the ancestor of the crown group (the crown ancestor) to its last common ancestor with a sister phylum. The crown synapomorphies are lost immediately. Some stem-group branches may be quite diverse, and their derived features may be so morphologically striking that in Linnean classification these branches would rank as extinct classes or orders of the phylum, ranks they can not be assigned in cladistic classifications. Proceeding toward the last common ancestor with a sister phylum, the constellation of features continue to erode. Eventually the morphologies at the nodes on the main line toward the common ancestor become dominated by features that are plesiomorphic. Finally, unique features of the phylum can no longer be recognized, and by definition the phylum is no longer present.
Stem-group taxa expand the morphological features and, usually, the morphological disparity of a phylum beyond that of a crown group, while at the same time they lack crown-group synapomorphies.
Although Valentine is referring to phyla, in the above quotes, the same idea applies to lower taxonomic categories. This raises a couple of interesting questions regarding human evolution. First, how can we tell the difference between stem group panins and stem group hominins or pre-divergence hominids? Second, how can we tell the difference between a member of the stem group and the common ancestor thereof?
Valentine (2004) On the Origin of Phyla
Wood and Harrison (2011) The evolutionary context of the first hominins, Nature 470:347–352. doi:10.1038/nature09709