Common Ancestor Terminology

A special note to preface: The criticism contained herein is not intended to challenge the fact of biological evolution. However, the terminology often used by public proponents does seem to be lacking in consistency with the natural world.When Charles Darwin entered the fray in the ongoing discussion about common descent, it was a discussion that already borrowed terminology from more mundane human geneaology. When social custom demanded worrying about property and violence and loyalty; a term like descendant and ancestor was intended to settle the matter once and for all. If Queen Bonnie owned Tulipsville in ye olden times and you could show the direct and real and sequential descendant of Queen Bonnie to all concerned; that was that, Tulipsville was yours. Transferred to evolutionary usage, if it could be determined and shown that the descendants of a small mammal that lived and loved in Cretaceous North America survived to present day as a human being living in Brooklyn, that would be that. Direct, Real, Sequential.On the other hand, going in the reverse order doesn't seem so convenient with the terminology at hand. Specifically, "the Common Ancestor" of, say, a human living today in Brooklyn and an African Elephant named Mogri living in the Serengeti today. Much can be inferred toward that end on a very general trend, more primitive primates here, and more primitive proboscidean there. And yet this only gets you so far. In the case of a descendant, you can safely throw out a great majority of the individuals as unimportant (not unimportant on ethical grounds, but just for the bookkeeping) , say the various siblings and such. But for an ancestor, if you try to ignore any of the individuals, you lose all of your accuracy. The closest you may be able to get is a population of potentially breeding organisms. The genes shared in common between a human and elephant of today did not necessarily come from an individual in that population, but could have been contributed by distant relatives in that population who are not connected in any direct linear fashion. The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more.Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature? Is there simply a weakness in the definitions of our language, and we need a new word? Or does the existing terminology need to be readdressed and properly defined to those outside the biological sciences?

Thread copied here from the Common Ancestor Terminology thread in the Proposed New Topics forum.

Hi Hideyoshi, and welcome to the fray, That's why I like to talk about a common ancestral gene pool, or a common ancestor population, rather than an individual. It is always useful to be more specific and define the terms as well as can be done, for clarity of understanding is necessary to meaningful debate.Enjoy

I'm not sure how this is meant to work. If you and I are related by being descended from two people who were cousins, then those two cousins must have had a common grandfather, and who is our common ancestor. Or if they were second cousins, they had a common great-grandfather, who was our common ancestor. Or if they were third cousins twice removed, then someone was the great-great-grandfather of one of them and the great-great-great-great-grandfather of the other ... and so on.In general, I don't see how you could have a scenario where two people (or elephants, or whatever) are related but don't have a common ancestor. What else is relatedness, when you get down to it?(Obviously for the purposes of this discussion, being "related by marriage" doesn't count.)So my brother and I have a common ancestor (my mother, to name but one); my father and I have a common ancestor (his father) my aunt and I have a common ancestor (my grandfather). My niece and I have a common ancestor (my grandfather again). My great-aunt and I have a common ancestor (my great-grandmother) ... and so on. You seem to be imagining that there's some degree of consanguinity by which two people (or elephants, or whatever) could be blood relatives but not have a common ancestor. Well, what is it?Edited by Dr Adequate, : No reason given.

Each cousin will also carry DNA from a set of grandparents that is NOT shared between the cousins. That DNA can be passed on to the next generation. An ancestral gene pool or population is a much, much better model.Let's stay with these cousins as our example. Let's say that a mutation occurs in one set of grandparents that is not shared. This mutation would end up in one of the cousins, and this potentially important genetic marker would not be the result of common ancestry between the cousins.Edited by Taq, : No reason given.

Yes, but that's another question.My question is, how can two people (or organisms) be related without having some individual who is their common ancestor? So far as I can see, they must do so simply by definition of "related" There has to be some point --- some individual --- at which their family trees meet, or they aren't related.The author of the OP seems to be suggesting otherwise. He says: "The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more." But surely he's wrong --- the closest that two organisms' family trees can get can never be distant cousinship, 'cos if they get that close then there must also actually be an individual common to both.Edited by Dr Adequate, : No reason given.

But the cousins in your example still share a common ancestor. If they didn't they wouldn't be related in any biological sense at all would they?

The author of the OP also said:"Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature? Is there simply a weakness in the definitions of our language, and we need a new word? Or does the existing terminology need to be readdressed and properly defined to those outside the biological sciences?"So this is really about how scientists use the term. What scientists are ultimately concerned with is how a species got the DNA that they have. For cousins, they share DNA from multiple common ancestors, not just one. Each mutation and gene will have it's own lineage within the population, and each could coalesce to a different common ancestor. With sexually reproducing species it makes no sense to talk about an individual as a common ancestor, at least from a genetic standpoint. We could also talk about incomplete lineage sorting and the impact it has on determining common ancestry.Ultimately, evolution comes back to genetics, and in genetics the common ancestor is a gene pool, not an individual.

They share multiple common ancestors from multiple generations, each of which made different contributions to their genome.

Yeah, but he's still wrong. No, not really. It would, of course be wrong to suggest that such an individual was unique, which I didn't. But they did exist. If humans and chimps (for example) have a species that is our common ancestor, then necessarily there was at least one individual in that species which was our common ancestor, because two individuals can't be related unless they have at least one common ancestor.

I fully agree that there would be such an individual, and probably more than one.But is that really what we are talking about when we reference a common ancestor? I would say no. That is not what we mean. When we say that humans and chimps share a common ancestor what are we really trying to describe? Well, we are ultimately talking about a speciation event where a single population split into two populations. The common ancestor would be the ancestral population that spawned the two separate lineages.When we look at the fossil record we are forced to use the same concept. We have no way of determining direct ancestor/descendant relationships between fossils, barring the discovery of DNA in those fossils. We are forced to describe relationships through common ancestry, and once again these relationships are referencing an ancestral population.The OP asked, "Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature?". That is a great question. Perhaps we SHOULD stress what we really mean by common ancestry. It is trivially true that we can trace the lineages of any two organisms back to an individual who is a common ancestor, but that is not really what we are describing. I think it also causes confusion when we talk about mitochondrial DNA and y-chromosome MRCA's. It can cause people to think that ALL of our DNA came from those two individuals.So should we be using something like "common ancestral population" instead of "common ancestor"? Perhaps we should consider it.

Well, we mean both. When we say that chimps and humans had a common ancestor, then we mean that there was some individual who was a common ancestor of both. And, of course, vice versa. We cannot say one without meaning (as a matter of logical necessity) the other.And this is the only point that I'm trying to clear up. Admit that I'm right, which I clearly am, and we have no quarrel.

My problem is my own biases in how I view it, so I would be happy to agree with that. I tend to stress the population over the individual common ancestor, but that doesn't necessarily have to be the case. You are mostly right.

And so would I, but not to the point of denying the individual common ancestor. That's all that I'm trying to clear up. Yeah, I mostly am.

I did not say they did not exist; but it's rather more like how we say a proton really and truly exists....but assure ourselves that a proton is not what we thought it was. Less a singular thing, and more a plurality of phenomena with a metaphorical center and boundaries.One of the problems with the terminology of human geneaology is that it is meant to track things like a last name...singular, distinct, either yes or no; and you object to it being treated any other way. But in the biological sense, these gene sequences are not that way. Organisms have offspring and those offspring have offspring...but many of the gene sequences they had do not, in fact, pass along. Moreover, a gene sequence may be present in both mating partners, but will only have been contributed to an offspring by one of the mating partners. Random gene sequence that allows for multiple muscle cell division in the snout; that was a mutation within a population of cretaceous mammals and not shared by the population, say.Again, this is not to say that common descent is not real; but rather that relatedness is not singular and distinct and literal to the individual as you intuitively think when you hear the term Common Ancestor.