Thursday, January 7, 2010

human origins and cladistics

Homo erectus?

Jacinta: When Tudge described Cro-Magnons as 'the first fully modern humans' I think he was referring to their cultural development, or evolution. My guess is that he was referring to fossil remains, in Europe, that were accompanied by other materials tools, perhaps ornamentations and such artifacts.
Canto: You mean there was no sign that these Cro-Magnons had evolved anatomically or physiologically from the Homo sapiens that they've managed to date back 500,000 years? By the way, can you give some more detail about this dating reference?
Jacinta: Tudge is a little vague on this. Here's the main quote:
The very first people who were more or less like ourselves – who would not have attracted too many stares on today's public transportation – date from about 500,000 years ago.
Canto: Oh so he doesn't specifically call them Homo sapiens. Or does he?
Jacinta: Well that's what I mean, he's a bit cagey. In fact one of the main take-home messages from this book is that we shouldn't get hung up on the 'Homo sapiens' moniker. As you know, there's been something of a shake-up in taxonomy over the past few decades, with clades and grades what they call cladistics being the preferred categories, with a lot of slippage and bushiness and imprecision in categorisation. Tudge doesn't give us a lot of detail on the fossils he's talking about from that era how much material has been found, for example - but he calls them 'archaic', or rather that's what the palaeontologists call them, but though they might be a little different from your modern Cro-Magnon type, they might not be different enough to prevent interbreeding. One definition of a separate species is that it breeds separately, though that definition is disputed, as is everything in taxonomy these days, it seems.
Canto: So we might be talking about the Neanderthals for example.
Jacinta: Yes, and there's another archaic type which has been given a separate species identity, Homo heidelbergensis, but Tudge doesn't provide any further information, whether it's based on half a skeleton or some jaw, or some teeth. Looks like I might have to read another book to get the dirt on that.
Canto: Or do some basic internet research.
Jacinta: We'll get on it soon.
Canto: So tell us a bit more about cladistics, if you can. I know we've talked about this before.
 Jacinta: Well, we're offline at present, because of stupid Optus downloading constraints, so my information will come from two books, The Link, by Colin Tudge, and Galileo's finger, by Peter Atkins. A clade is defined simply by Tudge as a group of creatures that shares a common ancestor. Clades exist within clades which exist within clades, but the key feature of a clade is this, to quote Tudge:
A clade is not a true clade unless it contains all the descendants of any particular ancestor plus the ancestor itself; and it must not contain any other creatures that are not part of the lineage.
This is clearly a tighter definition than what we have for a species, a definition that has changed over time and that has different emphases. For example, Atkins cites biological species, in which breeding pools are the significant factor, but also ecological species, recognition species and phylogenetic species, as well as the earliest understanding of species based on broad similarities. With the tighter approach of cladistics, we can create cladograms [a family tree or bush] which branch off whenever some significantly unique trait emerges. For example the loss of the opposable thumb on the foot occurred sometime after the chimps branched off from us some seven million years ago. That's a missing link, of sorts.