How many generations of Homo sp. and predecessors have had 23 pair of chromosomes?
**Thousands of years would convey similar information but I think generations would be more interesting.
Which definition of “generation” do you want to use?
18 years
20 years
25 years
33 years
50 years
70 years
Also, think millions of years, not thousands.
I’m no expert on human evolution, though I do find it interesting and have read quite a bit about it.
There’s a lot of evidence that human chromosome 2 is a fusion of two earlier chromosomes. The fused chromosome 2 has been found in us, Neanderthals, and Denisovans, which means that it is very likely that our common ancestor also had 23 pairs. That puts the chromosome fusion back at least 800,000 years, and maybe up to a few million years. If you go back much further than that, then you start getting into the common ancestor of us and other great apes, which did not have the chromosome fusion as the great apes all have 24 pairs.
So, it’s maybe somewhere between 50,000 and 200,000 generations (there’s different definitions for exactly how long a “generation” is, but I started with 20, you can do the math if you want to pick a different one). That’s a pretty wide range, but we don’t know a whole lot about our ancestors in that general time frame.
I’m confused.
Wouldn’t each member of every generation have to have the same number? If you have a different number you couldn’t have viable offspring (like 64-chromosome horses mating with 63-chromosome donkeys to produce sterile 63-chromosome mules), right?
But at some point we must have had a different number, because other animals have different numbers.
Again, this is not my field of expertise, but my understanding of it is that because chromosome 2 is an end-to-end fusion of two chromosomes, the two chromosomes that it came from can match up to it and create viable offspring. That allows one parent to have the two separate original chromosomes and the other parent to have the fused single chromosome.
It’s interesting to consider whether there was a time when humans could have either 23 or 24 pairs of chromosomes and still be an “average” prehistoric human. E.g. you could walk around a primitive settlement and the guy making arrowheads might be a 24-chromosomer while the ones churning butter by the riverside were mostly 23-chromosomers. Would chromosome count have functioned as a sort of racial identifier, perhaps? It’s likely that people didn’t recognize it, but might it have impacted appearance or behavior, or maybe even fertility? After some time, obviously, the 24-count humans would have died out for some reason. Did they get genocided in a prehistoric racial war? Slowly die out because the 23-count humans were stronger or smarter? Did they all get liberal arts degrees from Someday We’ll Invent the Wheel U and adopt a celibate life?
There must have been some time at which both were present. As for why 23 won out, it might just have been chance. It’d probably start during some population bottleneck, where something killed off most of a population, and just happened to leave more 23ers than 24ers. Once there were more of the new kind, as the population recovered, it was probably a benefit to be the new kind just because fertility rates are probably higher with the same number (even if the new fused chromosome can match up to the two old unfused ones, doesn’t mean it’s going to do so as well).
This may have gone in stages. For instance, the bottleneck might have only applied to a single tribe, and as they recovered, the 24s got culled out of that tribe. Then, that tribe competed with or went to war against other tribes, and by luck happened to prevail, and so on.
The importance of chromosome number in defining a species is wildly overblown in high-school level biology. As has been pointed out, if it were really all that important, it would be basically impossible for it to ever change.
Some terminology: we have two copies of each chromosome, of course. The two copies you have of any given chromosome are called homologs. Or homologues, depending on what side of the Atlantic you’re on. So you have two homologs of chromosome 1: one from your mom, and one from your dad. The same is true for the other 22 pairs. If you’re a male, your X and Y chromosome function as homologs, even though they’re not truly all that homologous. Having two homologs of each chromosome is “diploid”. Having only one copy of each is “haploid”.
OK. So, the ONLY point at which your homologs care about each other is during meiosis, which ONLY happens to make eggs and sperm. In your liver, kidney, skin, eyeball, whatever - the rest of your body - the cells are only undergoing mitosis, in which all of the chromosomes do their own thing. You could have 2 copies of each, or three, or fourteen, and mitosis would work just fine. (The cell would die of other problems, but not because it couldn’t do mitosis)
The point of meiosis is to reduce the cell’s chromosome number from diploid to haploid. Egg and sperm have to be haploid so that when they come together at fertilization, the resulting baby is diploid.
The key moment for this discussion is prophase of meiosis I. This is where your two homologs for each chromosome come together and pair up. Each chromosome HAS to find its partner and physically associate with it, so that when the cell divides, the homologs go in opposite directions and end up in different cells. If they fail to associate properly, they can segregate randomly, and you might end up with two homologs in one cell and none in the other. That would be BAD. That’s how you get Down syndrome, for instance.
All right. So, for meiosis to work properly, each chromosome needs to have a partner. Except…not really. What you REALLY need to have is a pairing partner for each chunk of DNA. That’s a key distinction for this discussion.
Imagine a hypothetical proto-human who got 23 chromosomes from his human-like dad, who has the chromosome 2 fusion, and 24 from his ape-like mom, who does not. What happens in meiosis when he tries to make sperm? Is he completely out of luck? Nope. Dad’s chromosome 2 has all of the sequences found in mom’s chromosomes 2a and 2b. So one arm of dad’s 2 can pair up with mom’s 2a and the other arm can pair up with mom’s 2b. The DNA sequences match up, so pairing isn’t a problem. Then they can orient properly on the spindle, and dad’s chromosome 2 will head to one cell, and mom’s 2a and 2b will head to the other. They can even go through crossing over and all that good stuff without any problems.
So it’s really not that big a deal. To make different species, it’s far more important what the sequence of the DNA is rather than how many pieces it’s broken up into.
As an aside, am I the only one who’s bothered by the terms “haploid” and “diploid”? The relevant numbers aren’t 1/2 and 2; they’re 1 and 2. Sperm and eggs should be referred to as “monoploid”, not “haploid” (or alternately, you could call them “haploid” and “monoploid”, but that would leave us without easy terminology for triploid, tetraploid, etc. organisms).
In my original post should I have been asking about ‘‘filial generations’’? I’ve some idea what I am thinking about but I am not sufficiently learned to know the technical terminology.
In the context of the question, I think it’s pretty clearly a desire to know how many parents in one’s ancestry one would have to go through to get to the 24-chromosome standard.
So somewhere in the 18-25 range for a generation.
I’ve long since stopped noticing, but yes, it’s quite confusing to students when you first teach it to them.
Well, except for one species.
Does Bubalus bubalis offer any insight? Domestic water buffalo, which all emerged in late neolithic times, can have either 48 chromosomes (swamp varieties) or 50 chromosomes (river varieties), yet are all considered the same species. Hybridization of the two strains is increasingly common, producing fertile 49-chromosome offspring.
No, I don’t think so. I think you wanted to know the plain number of generations–how many greats in the line of great great grandparents.
But the answer to your question is that we don’t know how many years average generation times were for our prehistoric ancestors. Some people have children at 15, some have children at 30, some have children at 45, and some (men) have children at 75. So in 150 years we could have 10 generations if everyone in the chain had offspring at age 15, or 2 if everyone had offspring at age 75.
We can get a better handle on just plain years, and you can do your own generational calculation based on whatever numbers seem good to you. The problem is that we don’t know the exactl number. It’s somewhere between 6 million years ago, and 800,000 years ago. The 6 million year figure is how long ago we believe 24 pair chromosome chimps and 23 pair chromosome hominids diverged. The 800,000 year figure is how long ago indisputable 23 pair hominids existed. When the 23 pair fusion mutation happened is sometime between then.
And of course, given that men can and historically often did have children at significantly greater age than women, the number of generations would vary significantly depending on which line you were looking at. Though most lines would have approximately the same number of male and female ancestors, so it’d mostly average out.
In my genealogical research, long term (on the scale of a thousand years) averages are much higher. I get about 30-33 years per generation.
People just didn’t marry and have most of their kids that early back then. (Remember, it’s the age when having the middle child that determines average generation spacing in this context.)
The real complication of the OP’s question is how long was a generation back in Homo Erectus, etc., days? Once you get back a few million years, was a generation under 10 years?
OTOH, molecular clocks, which can be used to approximate number of generations since species divergence do count actual generations.