Interbreeding is one aspect of defining species, but, as one might expect, it doesn’t tend to work all that well when examining fossil organisms. We really have no way of determining (short of a hypothetical time machine) which extinct organisms might be able to interbreed with other extinct organisms, or extant ones. So from a paleontological perspective, it becomes a matter of morphological differences which determine species groupings. There is, of course, a fair degree of “slop” inherent in such a methodology: how much morphological distinction is necessary for Fossil A to be considered “distinct” enough from Fossil B to warrant a new species designation? Or Genus? Or Order? It often tends toward more art than science, but, if the general consensus is that “yep, that’s a new species”, then yep, that’s a new species. It really comes down to how well a scientist is able to argue the case that two fossils belong to the same, or different, species.
I wouldnt call that “heavily” populated just more than today. ".…that may have supported as many as 50,000 people …"
OK, that being the case, it seems like the question becomes trivial to answer; any species that existed 55 million years ago will have undergone enough genetic drift that it will be not be the same species now?
Is that true across the boards? Cretaceous tardigrades would turn their noses up at a modern tardigrade’s chocolate and flowers? An elephant shrew from the Miocene would find today’s elephant shrews hopelessly unromantic?
This is amazing to me – I guess I was generalizing far too much from human behavior and our own species, with certain members thereof willing to mate with other humans, animals, and probably watermelons if carved comfortably.
Speaking of that . . . if I comb my hair, slap on some restroom Polo, and learn the lyrics to “Can’t Get Enough of Your Love, Babe,” how far back in time could I go and still be cross-fertile with our ancestors? (It’s not weird once you’re past great-great-great-great-grandmother, I swear).
(I know it seems like I’m ignoring your alternate grounds for species differentiation and focusing on fertility. Sorry).
(And let me just add that my late 1970s high school was totally lacking in this sort of informative discourse, so again, thanks all for bearing with my elementary questions. I am learning lots.)
55 million years is a good amount of time for tastes to change, even if the mechanics don’t.
Barry White should produce the desired results across all human existence.
If I found a cute Homo rhodesiensis 300,000 years ago, I’m telling you, it’d work. 
Unfortunately, 55 million is too far back to test the theory on humans.
Read up on the Red Queen Hypothesis. Back again to the tuatara, even though its gross appearance has changed relatively little over many millions of years, its genes are changing at a breakneck pace.
Given that Humans, Neanderthals, and Denisovians were interfertile, it stands to reason that the common ancestor of the three would be, too, which would take you back 400,000 years or so. Of course if you are so inclined, you could take a go at a chimpanzee or bonobo and see what happens.
Well, keep in mind that “willingness to mate” is not the same as “able to produce fertile offspring”; there are numerous examples (even within our own species) of, shall we say, attempts at cross-species mating. So an Olden Times tardigade may well still want to get it on with the latest model tardigrade, and vice-versa, but it’s probable that such a union would be fruitless.
There are a variety of things that can result in reproductive isolation, from behavioral changes to morphological changes, and even to simple geographic (or possibly even temporal, though we have no way of actually testing this) inaccessibility. Generally speaking, the former two processes are more likely than the latter when dealing with species separated by millions, or tens of millions, of years. Anything that keeps two populations separate allows the two to evolve independently, thus increasing the likelihood of reproductive isolation. So, not impossible per se, just incredibly unlikely.
The question “how far back in time could I go and still get a cavegirl pregnant?” can’t be answered without a time machine.
We’ve been genetically isolated from chimps for something like 6 million years. But species that have been isolated from each other for longer than that have been crossbred, like the famous llama dromedary hybrid: Cama (animal) - Wikipedia
Inter-fertility isn’t some on or off switch either. It’s a complex mix of geographic isolation, behaviors, fertility signals, species identifiers, physical compatibility, genetic compatibility, the survivability of the subsequent offspring, and the reproductive success of the subsequent offspring.
So could you get a Homo erectus chick pregnant? Maybe you could. You probably could. Would you want to? Maybe you’d be repulsed by her, and the reason you’d be repulsed is that she doesn’t have the same secondary sexual characteristics as a Homo sapiens sapiens. Closely related monkey species often have things like mustaches or weird fur patches that identify them as one species or another, members of one species highly prefer to mate with members of their own species, even if the resulting hybrids are perfectly viable. That Homo erectus female very likely could have a hybrid baby with a Homo sapiens, but there are pre-reproductive isolating factors that make that baby unlikely to be conceived even though it might be fully viable.
Get yourself a time machine, and lets find out.
The term “mate” means produce offspring, and of the fertile variety. It doesn’t mean “fuck”. Or, are you joking there?
Well, we don’t know for sure. We don’t even know “for sure” if we can’t mate with chimps even now. If you think about it, there are many large mammals that are considered to be different species (even different genera) that can interbreed successfully and which are more distantly related* than are humans and chimps. Think of camels and llamas, for instance. I’m not even sure if there are any large mammals that are as closely related as humans and chimps that are known to NOT be interfertile.
But there really isn’t any way of answering that question.
*meaning their evolutionary lines split longer ago than did the human/chimp line.
I prefer to say that I was deliberately eliding the distinction for comic effect.
Clarification: I was a bit sloppy with my use of the term “inter-fertile” (or however it’s spelled). When I was talking about chimps and humans, I was just talking about live offspring, not fertile offspring. Horses and donkeys split, evolutionarily, more recently than chimps and humans.
However, it may be more significant to look at the number of generation rather than the number of years, since humans and chimps have unusually long reproductive cycles. Or, to look at the actual DNA for a comparison.
And I was totally ninja’ed by Lemur about the camel thing. Those lemurs-- sneaky little primates!!
But let’s note here that phylogenetics is not a subjective matter. There is one objectively correct tree of life, for which the best evidence (by far) now derives from molecular phylogenetics, i.e. comparing DNA sequences among species to infer their evolutionary relationships.
Taxonomy is somewhat subjective, and a matter of convention/consensus to the extent that where we delineate taxonomic ranks is somewhat arbitrary. But taxonomy is subject to the objective constraints of phylogenetics, inasmuch as paraphyletic classification is avoided. In other words - it’s somewhat subjective what clade you consider constitutes a taxonomic rank; but it does have to be a clade.
For the record, ninjas and lemurs are pretty likely too far separated to be interfertile.
Agreed. But let’s keep in mind that it’s a tangled tree, and the branches often feed back on themselves before they permanently go their separate ways. So, if you’re talking about the human/chimp evolutionary split, you might have to ask: Which one? I’m not sure what the latest idea is, but it was certainly floated a few years back that we split once around 10M years ago, and then merged again for awhile about 6M years ago, and then split again. I could have the dates wrong on that, but there were (at least) two separate splits.
And to go back to the original question: could legends of dragons have been inspired by dinosaurs?
And the answer is no. The obvious reason that I gave earlier is that dragons were magic snakes. Dinosaurs aren’t snakes. QED.
If you showed a medieval knight a live Tyrannosaur, he wouldn’t think it was a dragon. He’d think it was a giant bird with teeth. And so would you, because Tyrannosaurs had feathers. Maybe they had big naked patches of skin, we don’t know what proportion of their body was covered by feathers or by skin. It wouldn’t look like a reptile to that medieval person, because the concept of “reptile” as a classification hadn’t even been invented. “Reptile” just mean a creature that crept on the ground.
Could some dinosaurs have survived the Cretaceous extinction? Sure, some did. But back in the Cretaceous there were all sorts of creatures with feathers that if they were alive today we’d call birds, even those that had teeth. But almost all of those creatures went extinct. It seems pretty likely that more than one species of bird survived the extinction, but all the surviving lineages of birds are fairly closely related. We have a few outsiders, like the tinamous, the hoatzin, and all the ratites. But all the diversity of bird types during the Cretaceous was largely wiped out.
And so are we going to find tomorrow a population of bird-like coelurosaurs in the Brazilian rainforest? Well, if we did the question is: why did this rare lineage survive as a rare lineage for 65 million years without either going extinct, or radiating into lots of new species? Why have we not found any fossils of these creatures? They were rare? OK, but if you’re a rare species for 65 million years you’re going to find yourself extinct. They survived only on an isolated island? OK, but which island? Where are they now? On top of some tepui? The thing is, those tepuis are not lush Mesozoic remnants. They’re a lot more recent than that, and they’re mountaintops. They’re mostly treeless. The tops look like this: http://images.summitpost.org/original/933464.jpg
I mean, it’s super cool, but there’s no hiding place for large herbivores up there, or large carnivores. There aren’t any dinosaurs up there, just like there aren’t any dinosaurs hiding in the New York sewer system.
Also, crocodiles. Today they’re all aquatic. But there used to be a lot more types of crocodiles running around, literally.
As for sea serpents: http://www.deepseanews.com/wp-content/uploads/2013/10/OarFish-big.jpg
OK.
If you really want to blow your mind, think about how we are really just all temporal ring species. If you couldn’t mate with a chimp, there is some ancestor of ours that you could mate with and who could also mate with a chimp’s ancestor and which could them mate with a chimp. Or, if not, there might be an ancestor you could mate with, who could then mate with another ancestor, who could mate with a chimp’s ancestor, etc.
And that applies to all living things. You are only x degrees of inter-fertility from all sexually reproducing species, whether it’s a chimp, a dog or a jellyfish. It’s just that, in our time period, we see only the outer branches of the tree of life. They all go back to one place, and we’re all part of the same, interconnected process of life on earth.
I’m not quite sure what you mean here. You seem to be implying that somehow a whole different genome would correspond to the same phenotype, and that’s not true at all. Natural selection operates on the information contained in DNA at the population genetic level.
It’s true that neutral mutations (DNA changes with no phenotype) accumulate steadily over time in parts of the genome that don’t do anything much. This is the “molecular clock” discussed earlier. So we could certainly detect the passage of 100 million years in the genome, regardless of phenotype.
However, if there is stabilizing selection for no change in the phenotype - i.e. the species is well adapted to its environment, and the environment is not changing - then the mechanism for this to occur is that the DNA does not change. In other words, any non-neutral DNA mutation (i.e. a DNA change that does have a phenotype) will result in lowered fitness, and that DNA mutation will tend to be elminated from the population.
That doesn’t contradict my point that the tree of life (however tangled) is an objective empirical matter, not something that is decided by a committee.
And I’m not quite sure what you are asking.
No, I didn’t mean it as a contradiction of what you said. Apologies if it sounded like that. I was expanding on the idea by noting that we usually think of branches on a tree as splitting off and then staying separate. But the branches on the tree of life are more complex than what we usually think of in a tree.