There is no missing link

Again, the press are talking about "the missing link". Let's get one thing clear. There is no missing link. Rather, there are an indefinite number of missing branches. To have a missing link, you need to visualise evolution as a chain. If there's a gap in the chain, then you have a missing link. But evolution, at least at the scale of animals and plants, is mostly a tree. And all we see are individual nodes of the tree, the extant species that form, in Darwin's metaphor, the leaves of the living tree, and the extinct species that form branching points deeper in the tree. But we do not have enough information to know the shape of the tree for all but the smallest twigs and larger branches. There may be, for all we know, millions of missing species. We might have a species that is an ancestor of some other species, and yet not know enough to say that they are indeed the ancestor in question.

This looks to be an exciting find, and possibly it will give us more information about the overall relationships of primates, but it is not the missing link, and it is one of potentially millions of missing nodes of the evolutionary tree.

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I think Hennig was right in saying that we cannot recognize ancestors; and that branching points in phylogenetic trees are "hypothetical ancestors". You cannot be sure that some fossil was a direct ancestor of any subsiquent species. Therefore, cladists do not understand the concept of "missing link". I do, however, see that geneticists are reconstructing ancestoral genes, which seems anti-Hennigian.

By Jim Thomerson (not verified) on 12 May 2009 #permalink

Jim: you assume ancestral states whenever you construct a phylogeny. How is that different from reconstructing ancestors?

Missing link schmissing link, all the same.

That is a good question. I see a difference in saying this is the plesiomorphous state of a character and saying that fossil A is the ancestor of Group A. One has to wonder; however, if we have fossil A which exhibits all of the plesiomorphous character states of Group A, maybe it is the actual ancestor. I think this is one of the problems those of my generation have had in accepting Hennigian assumptions. Cladistics is not a search for ancestors, and denies that they can be recognized as such. This is hard for many to swallow.

By Jim Thomerson (not verified) on 12 May 2009 #permalink

Well, yer actual fossil is unlikely to be a direct ancestor, fossils generally being individual organisms that have met an untimely end in some anaerobic and statistically unlikely fashion. I take your point though: there is of course the famous example of the horse family tree to show just how misleading fossil "ancestral forms" can be.

From an analytic point of view, I think if you don't accept some sort of ancestral reconstruction in the nodes of a tree, you can't take the tips seriously.

Of course I agree with your assessment of the "missing link" in scientific usage. However, the term has a more philistine meaning that essentially means a fossil finding that bridges a gap or has explanatory power. This is all personal opinion, but I think when Jane Q. Public is talking about the missing link, this is what they mean.

One point about reconstructing genomes is that there is a fundamental problem of minimum message length reconstructions that is formally analogous to the problem of reconstructing phylogenies - information is lost and can be reconstructed only on the basis of prior likelihoods. In other words, there are numerous ways that the present sequences could have been evolved, just as there are numerous ways the present taxonomy could have been evolved. We have to just live with the fact that the past is another country.

I fail to see how what we do not know about paleontology affects what we do know about neontology, Chris.

Ok, suppose you have two gene sequences from related species and you are trying to reconstruct the ancestral sequence. For each change, one or the other might be the changed one, or they might both be changed. Even if they are the same, they might have paralleled each other, and neither be the ancestral sequence. And that is not taking into account inversions, pseudogenes, and laterally transferred sequences. So the number of ways that a sequence alignment may have arrived at that point are manifold, indeed astronomical. Repeat and rinse for every gene in the species. The likelihood that the reconstruction is correct is vanishingly small.

To pare down that space of possible histories, just for two species, requires major assumptions and theory, and none of that can be independently supported except by analogy with other cases, and so the optimism Jim shows is, I think, unfounded.

Were them words easy to follow?

When idiots start talking about the "missing link", I deliberately misunderstand and tell them about the telomere block "link" in the middle of human chromosome 2, corresponding to the fusion site from primate chromosomes 2p and 2q. "See? The link has been found, so it's not missing."

This usually results in further incoherence, but at least the idiots learn one more little something.

Much more gooder, thanks John.

I'm a bit cynical about the approach that some molecular people have to evolution, so I guess I was thinking more of complex ancestral traits than gene sequences. Reconstructing ancestral character states (in the "organ that does X" sense) is something I've recently become interested in for the purposes of understanding how said traits arise.

I have coconspired with two different sets of DNA workers to generate phylogenetic hypotheses. In each case, I was sat down and bored to tears with an hour-long monolog about the problems and mistakes one faces in doing DNA. The lecturers were biologists first and DNA workers secondly, which may not be universally true. These experiences make me suspect that people trying to reconstruct ancient genes are well aware of the difficulties involved. I also suspect the problems John mentions are a small part of the total. Anyway, it is not something a good Hennigian would consider attempting.

By Jim Thomerson (not verified) on 13 May 2009 #permalink

See, now there I disagree with you. It's worth attempting. It's just not absolute truth (because information is lost over time). Phylogenetics is a preliminary step to reconstruction of the past, not the attainment of it, and all such reconstructions are tentative in direct proportion to the evidence available (phylogenies can rule out some histories) and the assumptions used in the models on which the reconstructions depend for their likelihood assignments.

Forgive me if i am incorrect for any of my comments because i am only fourteen years old and lack any experience in this field of knowledge, but i would like to say that the theory of a tree is quite accurate but could include more detail because it doesn't cover the branches of sub-species and the then the extinction of some sub-species and the seperation of a species and a sub species as that sub species becomes an entirely different species all together.

By Sanjay Naidu (not verified) on 20 May 2009 #permalink

If people really just have to have a missing link, Australopithecus afarensis is probably about as close to one as we're ever going to get.

By Peter Principle (not verified) on 21 May 2009 #permalink

I guess the idea is that the chain leading to humans
has nodes where other species branched off. The latest,
Neanderthals, branched off at H.Heidelbergensis.
In that sense, there is a missing link, in the sense of
no fossils, at the node where the Panini (chimps) branched
from the Hominini (humanoids), around 7ma. Not a big deal,
but finding such a fossil would help us understand the
beginnings of the hominins better.

Ida seems to be hyped way beyond credibility. The claim
in the popular press is that it represents a missing link
(a node where primates branched off). The claim in the
published paper is far weaker. The fossil in an
adapiform, which may or may not be the ancestors of the
Anthropoids (apes, Old World Monkeys and New World
monkeys).

Got a question here all,

It would seem that as large as the "Chain" of Human evolution would be it would be near impossible to find a "missing link". I understand the desire to study and search for our origins but would it not be better to study and put our efforts into learning how we are currently developing?

To be honest I am a "creationist" but I think there is no harm in having an open discussion, I have no desire to "debate" my own beleifs, being that they are based on pure faith something I find difficult to debate any way. I have always felt it important to get a clear picture of what the "other side" understands.

I also find it hard to doubt the fossil record, I can totally except that Homo Sapiens have "cousins" it has at times caused difficultlys with other "Creationist".

Any ways any response would be welcomed!

Thanks
Eric

By Eric Campbell (not verified) on 21 May 2009 #permalink

To pare down that space of possible histories, just for two species, requires major assumptions and theory, and none of that can be independently supported except by analogy with other cases, and so the optimism Jim shows is, I think, unfounded.