Some ideas one might think are pretty clear. The notion of an ancestor is one of them. But I am astounded how few people understand this simple idea in the context of evolution.
The basis for evolutionary thinking is the notion of an evolutionary tree, or a historical genealogy of species. It looks somewhat like the diagram in the header, which is a rendering of the first evolutionary tree from Darwin’s Notebooks. One species is the ancestor of another if it is lower in the tree diagram.
That seems simple enough, right? Well ancestry has a few wrinkles.
The first wrinkle is that an ancestral species of a modern, or extant, species might be extinct. Moreover, this means that if it is extinct, no similar modern species counts as an ancestor. One common misconception is that because our common ancestor with, say, lemurs looked like a mouse lemur, the modern mouse lemur is our ancestor. It is not. No matter how much a modern (or extinct!) species looks like another species, no matter how much resemblance there is, it is not the same species. Darwin was explicit about this:
We can clearly understand why a species when once lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur. For though the offspring of one species might be adapted (and no doubt this has occurred in innumerable instances) to fill the exact place of another species in the economy of nature, and thus supplant it; yet the two forms-the old and the new-would not be identically the same; for both would almost certainly inherit different characters from their distinct progenitors. [Origin of Species, 1st edition, page 315]
The idea that species are just groups of organisms that resemble each other was never a widely held view among naturalists before or after Darwin. Lamarck, and those who followed him, thought that all evolving lineages went through similar grades, and so that every existing species was “the same” as some extinct one, but that was never the view held by evolutionary biologists since Darwin. Ancestors were themselves the result of a unique historical process, and no more than Rome can reappear as the same empire it was in 64BCE can a species re-evolve.
A second problem with ancestors is identification. Often, both in the press and in some scientific texts, if something “looks like” it was an ancestor, then it was. This occurs most commonly in human evolutionary scenarios. Was Homo erectus our ancestor or was Homo ergaster? We do not know for a simple reason – we may not have yet found our ancestral species that immediately preceded Homo sapiens. In fact, we may never find it, as we haven’t found the ancestors of modern chimps, gorillas or orangs (though some think we have protogorillas). Since the absence of data is not data itself, we can only assess the probabilities – and this is not as easy as the rhetoric implies it is. It is likely that ergaster is an ancestor of Neandertals, but not of our present species. That is the best we can do. Possibly, given how difficult it is to identify a single species from paleontological remains, ergaster is not even a real species. It might be part of a much wider species that included erectus, and we might be part of that species as well (with some modifications that don’t inhibit interbreeding if only there were some individuals around to experiment on). Or it might be that all the individuals that are called erectus or ergaster are in fact members of many smaller species, but the evidence of that is not fossilised and we’ll never know for sure.
“Cryptic” species, which resemble each other so much even experts can’t tell them apart unless they are observed not to interbreed, are common. “Polytypic” species, where the range of morphological variety is so great, as they are in domestic dogs, that individuals would almost vertainly be classed as different species if not for the knowledge that they do interbreed, are also common. So working out what is a species, let alone what is an ancestor, is very difficult and ultimately, I think, impossible to know for sure.
Even in a famous case like Darwin’s finches on the Galápagos Islands, which are thought to have come from a species still extant in South America, we cannot identify the parent species because it might have been one of five or six, and the evidence doesn’t narrow it down. We know it was one of them (or maybe a closely related but now extinct and unpreserved species), but finding the culprit is harder than proving O.J.’s guilt to a jury. All kinds of likelihoods, but no knockdown evidence.
A third confusion is the notion of “the” ancestor. Every species alive has literally thousands, if not millions, of ancestral species going back to the evolution of the major group – Animals or Plants or Fungi, to name the three larger eukaryotic kingdoms. Each ancestor forms a node on a phylogenetic tree, and there are a multitude of these branching points. Basically, an “ancestor species” is a species that gave rise to at least two modern or later species.
Some useful terms – the cenancestor is the last ancestor shared by all life. The concestor, a term introduced in Richard Dawkins’ recent The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution, is the last species that is in the lineage of any two modern organisms. So, the concestor of humans and chimps lived about 5 or 6 million years ago, while the concestor of us and, say, brewer’s yeast, lived around one and a half billion years ago or so. The concestor of all life, the cenancestor, lived around 3.85 billion years ago.
A fourth confusion is the notion of “more evolved” species. As demonstrated by the recent media meme of chimps being “more evolved” than us, no species is more or less evolved except in some specific sense, like (as in the chimp case) the number of selected genes over a period. All species at a given time have exactly the same evolutionary duration, and on average, probably the same number of ancestral species, as their nearest relatives. And “primitive”, as I have noted before, doesn’t mean more complex or “more perfect”. It simply means that the ancestor has fewer of the changed traits of its descendants, and more of the general traits of the group as a whole.
So, I hope this brings some clarity to the notion of an ancestor. We all have them, but we don’t all know who or what they are, or talk about them over dinner.