How many ancestors do I (we) have?

Factual Questions
1

Thinking about evolution today - and maybe being a bit depressed & trying to cheer myself up, I got to thinking. Every single one of my ancestors all the way back to some primordial goo, has successfully survived to the point where they could procreate. Else, I wouldn’t be here. That’s a lot of success in my family tree! Think how many didn’t make it!

Anyone ever figure out roughly how many ancestors/generations I have? Going all the way back to the primordial goo/soup?

Were any of my ancestors dinosaurs? In that case, “great extinction” my butt!

Lev

2

Another of those great unanswerables. The vast majority of species which have ever lived are not accounted for in the fossil record. Even of the ones which are present, there still exist gaps between species A and species B. Even where the gaps are relatively small, we don’t know how many generations passed between species X and species X’.

So, the unfortunate answers to your questions are “no” and “no”.

**

Not unless you’ve got feathers.

3

Cecil’s column on ancestors is a good starting point. In another column, he tells us that “Demographers have come up with estimates ranging between 69 billion and 110 billion humans” having ever existed. Looks like you’ve got your work cut out for yourself on that family tree of yours.

4

Strictly an estimate: there are 3-4 generations of humans per century. (I base this on my great-great-grandfather, who died 101 years before I was born.) Let’s use 3 [sup]1[/sup]/[sub]2[/sub] per century for an average.

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.

So, going back 2 generations gives 2[sup]2+1[/sup]-2 = 2[sup]3[/sup] - 2 = 8 - 2 = 6.

Going back to New Testement times is 20 full centuries or 70 generations. 2[sup]70+1[/sup] - 2 = 2[sup]71[/sup] - 2 = 2,361,183,241,434,822,606,846. ( 2 sextillion, 361 quintillion, etc.)

There obviosly haven’t been that many humans ever in existance, but there are that many branches in your family tree. There are mostly duplicates at this level, so were all basically inbred if we look back far enough. :D:D

And this was just for the last 2,000 years. Mankind has been around a couple million years, our generations were closer together, and the previous species we evolved from most probably had very close generations too (probably closer).

Once we get back to sea creatures, our generations were probably only a year or two apart. Back to simple multicellular creatures, there were probably a few generations per year. Back to single-celled creatures, there were probably several generations per day.

As to whether we are descended from dinosaurs: unlikely, probably no. Mammals probably descended from shrew-like creatures that co-existed with the dinos. They were too small to be fodder for large carnovores and prolific enough to not be wiped out by those that did eat them. And when whatever event that killed the large dinos happened, the proto-mammals were ready to fill all the evolutionary niches left empty. Birds are most probably the descendants of dinosaurs. (Not the large lumbering ones like triceratops and bracheosaurs, but the two-legged raptors.)

5

Whew! I knew the numbers would end up being astronomical. Thanks for taking a stab at it. What really impresses me is no matter how many generations it took to end up with me, every single one of my ancestors had to survive at least long enough to reproduce. All the natural disasters, asteroid strikes, predation, fratricide, disease, murder, war and somehow I/we made it. From some sort of primordial organic self-reproducing goop to single cell organisms all my predecessors made it through all the varieties, species and eventually to me. Every single one of my predecessors was a success. For billions of years! Doesn’t that make winning the lotto seem like a sure thing?

OK, I could have researched that myself I suppose. :smack: I was thinking some where between sea creature and shrew like mammal there might have been something dinosaurian. Not big dinosaur, mind you.

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?

Well I still have to feel pretty lucky and feel a lot of pride all my ancestors survived billions of years of survival of the fittest.

Lev

6

Of course, if this sequence of successes did not occur, you wouldn’t be here to notice how amazing it is that this sequence of successes occurred.

Though, you can miss out on a win in the lottery and notice that you lost.

7

You also need to remember that any sequence of ancestors that ends up with anyone is just as equally unlikely (or likely).

So if you weren’t here to marvel at the chances, some equally unlikely person would be.

8

I almost hate to bring this up because it boggles the mind, but for every single birth in your (our) family tree there were 300 million sperm cells competing for the opportunity. Talk about astronomical numbers! — Actually, it kind of makes you feel special, don’t it?

9

Now would be a good time to introduce the anthropic principle hey?

Anyway, Ill do a rough estimate and assume that the bulk of your ancestors were actually single celled organisms rather than bigger animals. Now, assuming that you actual ancestors managed to sit on hte bottom of a pond for a billion years. Now assuming you have maybe a cubic meter of bacteria. this site claims that there is 9 trillion bateria per cubic meter so 5 x 10 ^17 beacteria in the pond. Assuming each bacteria survived for a day, that would mean a total of 2x10^29 ancestors.

10

Oddly, I find it comforting that back in my family tree there were ancestors who just couldn’t get laid enough.

11

"… in each human coupling, a thousand million sperm vie for a single egg. Multiply those odds by countless generations, against the odds of your ancestors being alive, meeting, siring this precise son; that exact daughter, until your mother loves a man … and of that union, of the thousand million children competing for fertilization, it was you, only you, that emerged. To distill so specific a form from that chaos of improbability, like turning air to gold … that is the crowning unliklihood.

The thermodynamic miracle.

But the world is so full of people, so crowded with these miracles that they become commonplace and we forget. We gaze continually at the world and it grows dull in our perception. Yet seen from another’s vantage point, as if new, it may still take the breath away.

… For you are life, rarer than a quark and unpredictable beyond the dreams of Heisenberg."

– Doctor Manhattan, Watchmen

12

AWB, I agree with what you’re saying here. One point that I’ve wondered about occasionally is how many generations have passed since the first upright hominids decided to take a stand. The 3-4 per generation number seems right for recent centuries, but keep in mind that even a within the last millennium people were considered adult and married younger than they do today. Perhaps 5-6 generations per century up until around 300 years ago would be more accurate?

13

[QUOTE]
*Originally posted by levdrakon *

Whew! I knew the numbers would end up being astronomical. Thanks for taking a stab at it. **What really impresses me is no matter how many generations it took to end up with me, every single one of my ancestors had to survive at least long enough to reproduce. ** All the natural disasters, asteroid strikes, predation, fratricide, disease, murder, war and somehow I/we made it. From some sort of primordial organic self-reproducing goop to single cell organisms all my predecessors made it through all the varieties, species and eventually to me. Every single one of my predecessors was a success. For billions of years! Doesn’t that make winning the lotto seem like a sure thing?

[/QUOTE

Bolding mine.

Well with sperm banks in full force and cloning in the extremely early stages of bringing a Brave New World, will our future generations be saying this about us and our children or will they merely be referring to us as the maturbator or dna/genome bearer? :smiley:

In todays world you could be gone tomorrow but your family line can go on without you. One day, if it hasn’t already happened, a virgin man will be a biological father. Pretty weird eh? But not very biblical. :smiley:

14

Not really. From my genealogy I get 34 generations/last 1000 years. And it’s only with recent centuries (1800 and 1900s) that the average generation time decreases. Keep in mind that far larger effect on the average was that people had kids quite late, women until at least 45 (if they lived that long) and men of course still later (esp. with 2nd, 3rd, … wives). So while a few of your ancestors may have been born to young parents, many were born to middle aged ones. Among my lower class European ancestors, a man generally didn’t get married until he had accumulated enough “wealth” to do so. That takes time. Quite a few people of the time never married since they never had the where-with-all to do so.

15

Hmm. Okay, I hadn’t thought of it that way. I always thought there was a societal urge to have as many kids as possible, to ensure some would survive, but I guess that wasn’t universal. Maybe it wasn’t even really common.

16

Out of millions of sperm, I was the fastest!!

17

Actually, the technology for virgins to become fathers or mothers is very simplistic, and probably could have been done centuries ago.

Two words: Turkey Baster

18

Come on, let’s not wimp out and stop at hominids. The OP wanted to go back to primoridal ooze, so, by God, back to primordial ooze we shall go.

Here’s a list of our ancestors at each stage of our evolution, along with my best estimates for (a) the number of years in each stage, (b) the average length of a generation for each class of organisms, and © the product, or total number of generations in each stage:

  1. Humans/hominids, 5 million years, 20 years, 250k generations
  2. Proto-apes, 15 million years, 10 years, 1.5M generations
  3. Lower primates, 40 million years, 2 years, 20M generations
  4. Early mammals, 200 million years, 1 year, 200M generations
  5. Mammal-like reptiles, 100 million years, 1 year, 100M generations
  6. Amphibians, 40 million years, 1 year, 40M generations
  7. Fish, 100 million years, 0.5 years, 200M generations
  8. Proto-chordates, 100 million years, 1 month, 1.2B generations
  9. Single-celled animals, 400 million years, 1 day, 146B generations
  10. Bacteria, 2.5 billion years, 1 day, 912.5B generations

Total, a little over a trillion generations, mostly bacteria.

I see no way of even estimating the number of individual ancestors at any point in history. It depends entirely on the size of populations which spun off to form new species at key branching points in our evolution. The only answer I can give, because of this branching process, is “a lot fewer than you might think.”

19

There are a few problems with the above list, not the least of which is the fact that amphibians do not make up part of our ancestry.

Other issues:
The oldest sarcopterygian fishes (the group of fish which actually led to us) are about 390 million years old. The earliest “fish-like” things (known as “thelodonts”) which would eventually give rise to sarcopterygians are from about 440 million years ago. Prior to that, you get into “proto-chordates” and the earliest craniates (things with heads). So, realistically, this drops the distance from steps 1 to 7 to about 440 million years in total (with step #6 excised, of course).

The oldest known synapsid (aka, “mammal-like reptile”) is from about 320 million years ago. This knocks about 40 million years from the time from steps 1 through 5.

The earliest known mammal of the group that eventually led to us (Eutheria, or placental mammals) is from about 125 million years ago. The oldest known mammal fossil is about 195 million years old. Which means the total from steps 1 through 4 should be about 200 million years; the duration of step #4 needs to be reduced by about 50 million years as a result.

Along those lines, the oldest known primate is from about 40 million years ago. The duration of that step (#3) needs to be likewise reduced by about 20 million years.

Incidently, I found this article over at Talkorigins, which may be of interest and/or use.

20

Then how should the earliest tetrapods, which laid jelly-covered eggs and from whence reptiles evolved, be classified?