Well, as Tetrapods (clade Tetrapoda). The tetrapod group includes the most recent common ancestor of amphibians and amniotes (which in turn gives rise to reptilians and our synapsid ancestors).
In real life, you certainly do not have twice as many ancestors for each generation back you go. The phenomenon of “pedigree collapse,” well known to genealogists, cuts down on the number of your ancestors. (See The Mountain of Names by Alex Shoumatoff for more on this.) It’s caused by cousing marrying their cousins.
If your parents were unrelated, you have four different people as your grandparents and eight different people as your great-grandparents.
But if you marry your first cousin (don’t squirm, it happens all the time in many societies), you will have only 6 grandparents between you instead of 8. So if your parents were first cousins then you have only 6 great-grandparents instead of 8. The more that cousins marry cousins, the more this ancestor reduction is cumulative.
Shoumatoff writes (p. 230-231)
In olden days, the pool of available mates was limited to a small geographic radius. Practically everybody was marrying their second or third cousins. So each person’s family tree is shaped like a diamond: it widens out to a point, but as you go back further it narrows.
Actually, it is quite certain that we all go back to ONE single-celled organism that lived something like 3.8 billion years ago! And no we are not descended from dinos. Birds seems likely to be (that is somewhat controversial), but we are not. We are however, descended from a fish-like ancestor called a lungfish. They differred from the main line of fishes in having something that could develop into a lung and are the ancestor of all land vertebrates (as well as cetaceans and any other air-breathing creatures).
Somewhere, some biologist or paleontologist (Stephen J. Gould?) wrote something very like what the OP said about the fact that each of us is the result of an incredible series of improbabilities that together make each of us the recipient of an unbelievable run of luck. Most species die out; most individual lines die out.
It’s not all that controversial. There are still some holdouts (e.g., ornithologist Alan Feduccia) who don’t buy the bird-> dinosaur transition, but most do.
Recent evidence suggests that tetrapods share a common ancestor with lungfish. Which means lungfish aren’t part of our lineage.
[ul]NO but you’ve got a shot at FRED or BARNEY[/ul]
Ok, tetrapods are a subgroup of the Sarcopterygii, which also includes several groups of lobe-finned fish, such as lungfish and the coelacanth. So my ancestors had four legs. Then what? There was never anything dinosaur-like?
No, levdrakon, no dinos on the family tree–unless you’re willing to count anything reptilian as “dinosaur-like”. But as long as this thread has been resurrected, I’d like to ask a follow-up question, if Darwin’s Finch is still reading. Here are two pairs of statements:
A1. Humans descend from amphibians.
A2. Humans and amphibians share a common ancestor, which was neither human nor amphibian.
R1. Humans descend from reptiles.
R2. Humans and reptiles share a common ancestor, which was neither human nor reptile.
Now, from your correction to my earlier post, I gather that the correct statement in each pair is A2 and R1.
Why is this? Are “early fossil tetrapods” more different than modern amphibians than early fossil reptiles are from modern reptiles?
Also, which is the correct statement with respect to (a) apes; and (b) fish?
I’ll give you a rough estimate…Assloads is a term I think applies.
No dinosaur-like critters in our ancestry. Pity, really.
A quick-and-dirty breakdown of our lineage is as follows:
Tetrapods begat the Reptilomorpha, which begat Amniota, which begat Synapsida, which begat Therapsida, which begat Cynodontia, which begat Mammals. There are a number of esoteric subgroupings between each step along the way, but the ones listed are the highlights.
Actually, it’s A2 and R2 - that is, humans and reptiles share a common ancestor (as amniotes), and humans and amphibians share a common ancestor (as tetrapods). Our branch of the tetrapod line is known as Reptilomorpha, and the amphibian branch is the Lepospondyli. Our branch of the amniote line is Synapsida, and the reptilian branch is Sauropsida.
Early fossil tetrapods were pretty nondescript, as far as “things that can walk on land” go. Modern amphibians are fairly similar to those early tetrapods, but still have many features not found in those early critters (like the forms of the teeth, vertebrae, and hearing apparatus). Modern reptiles share many similarities with early versions; they are all reptiles, after all. The main differences would be between the various sub-groups of reptiles, rather than between early and late versions.
I’m not sure I understand the question…are you asking how the above questions would be answered if you substitute “apes” or “fish” or “humans”?
And, should you really be interested, you can read this annoyingly lengthy post as to why dinosaurs weren’t typical reptiles.
[QUOTE]
*Originally posted by Darwin’s Finch *
Our branch of the tetrapod line is known as Reptilomorpha, and the amphibian branch is the Lepospondyli. Our branch of the amniote line is Synapsida, and the reptilian branch is Sauropsida.
Just so I’m clear on this, then–if one were to consider one of these early Reptilomorphs, wandering around some 340 million years ago, before the Sauropsid-Synapsid split–that it would not be correct to describe such a creature as a reptile? Even though a lay person looking at it would probably say, “That looks like a reptile to me.”
[QUOTE]
I’m not sure I understand the question…are you asking how the above questions would be answered if you substitute “apes” or “fish” or “humans”?
Consider the most recent common ancestor of humans and chimpanzees. Would it be correct to describe this creature as an “ape”?
Likewise with fish. Consider the most recent common ancestor of humans and today’s fish–was this creature a fish?
Early Reptilomorphs were what could certainly be described as “lizardy-lookin’”. However, they would technically not be reptiles. At least not in any taxonomic sense. Note that the pre-split reptilomorphs did not possess an amniote egg - an important step for both the reptilian line and the synapsid line. So, in many ways, they were more like “lizardy-lookin’ amphibians”, while being neither an amphibian (in the Lepospndyli sense) or a reptile (in the Reptilia sense).
The most recent common ancestor of chimps and humans would, indeed, still be an ape.
The fish question is a bit more complicated, since in taxonomic terms, “fish” is somewhat meaningless. There are two types of creatures which are generally referred to as fish: the actinopterygians (ray-finned fishes - the kind we typically associate with the word “fish”) and sarcopterygians (fleshy-finned fishes - like the coelacanth and our pre-tetrapod ancestors). Together, actinopterygians and sarcopterygians constitiute Osteichthyes. Osteichthyes, of course, were pretty “fishy” in form and habit. In Linnaean taxonomy, they represent their own class, whereas in cladistic taxonomy, they represent our shared ancestors with typical fish. So, in the vernacular sense, the most recent common ancestor of “fish” and “humans” would still be a “fish”; it just wouldn’t be a fish quite like any modern fish.
For that matter, the most recent common ancestor of myself and my sister is an ape (well, two apes). Humans are not just decended from apes: We are apes. How easy it is to forget.
I guess that makes sense, since humans and chimps are more closely related than chimps and gorillas. So a classification of “ape” which included gorillas and chimps, but not humans, doesn’t make any sense. Hmmmm . . .
Indeed! Such is one of the premises of cladistic classification: any valid taxon must contain an ancestor and all of its descendants - such a taxon is considered to be monophyletic. Any group which does not include all descendants is paraphyletic, and does not represent a “natural” taxon. Thus, we are apes as well as cynodonts, therapsids, synapsids, amniotes, reptilomorphs, tetrapods, etc.
[aside]This is one of the main problems with Linnaean taxonomy: it provides little insight with regard to the ancestor-descendant relationships of organisms by recognizing paraphyletic taxa. Aves, Osteichthyes, Reptilia and Mammalia, for example, are all given the equivalent rank of “class”, which completely obfuscates the actual relationships between these groups.[/aside]
Read this very fascinating article recently published in The Atlantic, cousin: