# How many ancestors do I (we) have? **URL:** \ **Category:** Factual Questions\ **Created:** [November 13, 2002, 2:50am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524 "2002-11-13T02:50:40Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 16, 2002, 7:39pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/21 "2002-11-16T19:39:45Z")
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). ---
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [November 16, 2002, 9:19pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/22 "2002-11-16T21:19:16Z")
> [@](#): > > \*Originally posted by AWB \* > **Mathematically, you have twice as many ancestors for each generation back you go; i.e., your parents are 2 ancestors, your grandparents are 4 ancestors, etc. Summing these for _n_ generations: 2[sup]1[/sup] + 2[sup]2[/sup] + [sup]3[/sup] + … + 2[sup]_n_[/sup] = 2[sup]_n_+1[/sup] - 2.** 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) > [@](#): > > …if you kept multiplying your progenitors by two every generation—doubling your parents, _their_ parents, and so on—when you reached the time of Charlemagne (ca. 800 A.D.) you would have between four and seventeen billion of them. Each person’s pedigree, in other words, experiences a sort of retrogressive Malthusian population explosion. Gunderson has since revised his estimate, claiming that if you kept doubling every twenty-five years the pedigree of someone born today, by the year 800 the final number would be more like two hundred and sixty-three trillion. > > But obviously there were nowhere near that many people alive then, or at any time. … Pedigree collapse is caused by cousins marrying cousins—intentional mating between close cousins and random mating between distant ones who don’t even know they are related. … > > “If we could get into God’s memory,” Robin Fox told me recently, “we would find that eighty percent of the world’s marriages have been with second cousins. In a population of three to five hundred people, after six or so generations, there are only third or closer cousins to marry…” 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. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [November 17, 2002, 2:00am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/23 "2002-11-17T02:00:02Z")
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. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 17, 2002, 6:11am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/24 "2002-11-17T06:11:10Z")
> [@](#): > > \*Originally posted by Hari Seldon \* > **Birds seems likely to be (that is somewhat controversial), but we are not.** 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. > [@](#): > > **We are however, descended from a fish-like ancestor called a lungfish.** [Recent evidence](http://www.bulletin.ac.cn/ACTION/2002090601.htm) suggests that tetrapods share a common ancestor with lungfish. Which means lungfish aren’t part of our lineage. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [November 17, 2002, 7:00am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/25 "2002-11-17T07:00:47Z")
> [@](#): > > \*Originally posted by levdrakon \* > \*\*Were any of my ancestors dinosaurs? In that case, “great extinction” my butt! Lev \*\* [ul] **NO** but you’ve got a shot at **FRED** or **BARNEY** [/ul] ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [November 26, 2002, 5:12am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/26 "2002-11-26T05:12:48Z")
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? ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [November 26, 2002, 5:36am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/27 "2002-11-26T05:36:18Z")
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? ---
**Author:** ![Critical1](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@Critical1](https://boards.straightdope.com/u/Critical1)\ **Post date:** [November 26, 2002, 5:55am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/28 "2002-11-26T05:55:55Z")
> [@](#): > > \*Originally posted by levdrakon \* > \*\* > So astronomers can figure out about how many of the “billyuns & billyuns” of stars might contain planets with life, yet we have no idea how many generations led up to us? Bummer. Of course I know we’re missing a lot of intermediate species back in our family tree. But no one has made even a rough guesstimate? > > \*\* I’ll give you a rough estimate…Assloads is a term I think applies. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 26, 2002, 6:39am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/29 "2002-11-26T06:39:41Z")
> [@](#): > > \*Originally posted by levdrakon \* > \*\*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 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. > [@](#): > > _Originally posted by jklann_ > \*\*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?\*\* 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. > [@](#): > > **Also, which is the correct statement with respect to (a) apes; and (b) fish?** 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](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=132981) annoyingly lengthy post as to why dinosaurs weren’t typical reptiles. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [November 26, 2002, 9:42pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/30 "2002-11-26T21:42:21Z")
[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? ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 26, 2002, 10:23pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/31 "2002-11-26T22:23:02Z")
> [@](#): > > \*Originally posted by jklann \* > **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."** 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). > [@](#): > > \*\*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? \*\* 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 26, 2002, 10:52pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/32 "2002-11-26T22:52:47Z")
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. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [November 26, 2002, 11:36pm UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/33 "2002-11-26T23:36:53Z")
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 . . . ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 27, 2002, 1:34am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/34 "2002-11-27T01:34:39Z")
> [@](#): > > \*Originally posted by jklann \* > \*\*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] ---
**Author:** ![Alear](https://avatars.discourse-cdn.com/v4/letter/a/c6cbf5/32.png) [@Alear](https://boards.straightdope.com/u/Alear)\ **Post date:** [November 28, 2002, 1:23am UTC](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524/35 "2002-11-28T01:23:41Z")
Read this very fascinating article recently published in The Atlantic, cousin: > **[The Royal We](https://www.theatlantic.com/magazine/archive/2002/05/the-royal-we/302497/)** > > The mathematical study of genealogy indicates that everyone in the world is descended from Nefertiti and Confucius, and everyone of European ancestry is descended from Muhammad and Charlemagne [Previous page](https://boards.straightdope.com/t/how-many-ancestors-do-i-we-have/136524.md?page=1)