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suvlub t1_j4zu2gj wrote

Reply to comment by CoolioMcCool in Given that reproduction is difficult or impossible when both animals have different numbers of chromosomes, how did so many species evolve to have so many different numbers of them? by MercurioLeCher

Yes. Their children, however, would end up with 45 chromosomes, which would make it difficult, but not impossible, for them to reproduce. Their family has a long history of miscarriages, unfortunately.

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Slashy1Slashy1 t1_j50e1rq wrote

Right, and that seems to illustrate OP's point. Having 44 chromosomes is obviously a pretty big fitness detriment, since it makes it harder to reproduce with other members of your own species. So how did a such a variation in chromosome numbers between species occur in the first place, if evolving it is a detriment?

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thegreenrobby t1_j50mx28 wrote

Evolution isn't a perfect system. It's a game of repeated "good enoughs". If the genes with a disadvantage get a little lucky with their reproductive odds during the initial generations of the mutation, there's no reason a fully detrimental mutation might not stick around for a while.

Also, humans tend to be the exception to a lot of rules. Our knowledge of medicine (Edit: and agriculture, and a buncha other things) significantly alters our fitness odds, and allows many genes to reproduce that may not have otherwise survived.

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HornedDiggitoe t1_j50sxod wrote

> Our knowledge of medicine significantly alters our fitness odds, and allows many genes to reproduce that may not have otherwise survived.

Knowledge of medicine can’t be credited for that. That was largely due to human knowledge of farming/agriculture, as well as human empathy to care for the weak. Humans not surviving long enough to reproduce was historically caused more by a lack of food than anything else. If you had a dead weight (disabled) human in your group and not enough food to go around, guess who isn’t going to get fed?

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rising_ape t1_j50xhr4 wrote

Interestingly, our empathy "to care for the weak" may be more important than our knowledge of farming and agriculture here - we've found Neanderthal fossils that were severely disabled in life and would have been unable to care for themselves, but whose bones reveal that their initial injuries healed and that they lived on for years despite being "dead weight" (physically, at least).

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HornedDiggitoe t1_j50xrqe wrote

Right, but that would’ve only been possible if the Neanderthals had enough extra food to feed themselves and the disabled Neanderthal. You don’t necessarily need agriculture to have an abundance of food, but it certainly helps a tonne to make food abundance widespread.

It still circles back to being about food in the end.

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rm_systemd t1_j50yrtm wrote

It definitely can. Statistically, most people died in childhood from the many fevers, whooping cough, tuberculosis, syphilis, measles, polio, malaria, and infected wounds. There are even more tropical diseases, which is why Europeans had a life expectancy of 1 year in Central Africa prior to their discovery of Quinine.

Adult women then had to chance the maternal death rates due to hemorrhage and puperal fevers.

Those are the greatest reasons behind the 32 year life expectancy.

You can also credit sanitation, agriculture and industrialization, but vaccination soon after birth is mandatory for a reason, and that is why we had a way higher population than what ancient Rome and China could support, even with their excellent infrastructure and decent agricultural capacity.

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HornedDiggitoe t1_j514owa wrote

Without food abundance none of that medicine would have helped much. All these medical marvels you brought up were invented after agriculture. Imagine what the life expectancy was for disabled/sick people prior to an abundance of food.

Also, 32 years old is old enough to have reproduced and pass on genes. Life expectancy was much lower prior to agriculture.

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rm_systemd t1_j52wsn7 wrote

In ancient China, food was not more abundant. In fact, everyone outside of the top 2% ate mostly unpolished grains and wild vegetables, and were usually about 5 feet tall due to poor nutrition. However, Chinese medicine was effective as preventative medicine and supportive treatment, and so the empirical evidence stands that their cities were historically the largest until the industrial revolution entered full swing.

Farming in China has been largely unchanged for the last 2600 years, they had very little arable land per capita and no access to the abundance of the sea like Japan does. Rice is also a luxury for most of history, and only a staple in the South. Northern China was fed on wheat, millet and sorghum etc., and the Yellow River is the area that the Han culture originated and thrived for most of history.

Your point about feeding the weak only applies to famine and war, in a time where death rates are already high. It won't be statistically significant then, because everyone would be hungry and weak, then the plague or a hostile army would come out of nowhere and flatten them anyway. In that case, survival was as much luck as it was rational decisions.

The family, tribe or clan was also the most important unit in all of history, and they always provided for the infirm. Even Neanderthal tribes have left behind evidence that they supported the disabled. Liberalism was significant, because it recognized the individual, where the traditional conservative only saw clans as the smallest unit. That is not how it worked for the longest time. If you were family, you just fed them, it was that simple

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Emu1981 t1_j52xrle wrote

>Imagine what the life expectancy was for disabled/sick people prior to an abundance of food.

What makes you think that there was no abundance of food before the discovery of agriculture? Hunter gatherer groups tended to migrate around to follow the food over the seasons. Between this and the low populations it would have been pretty rare for the groups to go hungry over a long enough period of time for individuals to starve to death.

Agriculture and animal husbandry is what allowed for humans to settle down and to start multiplying like rabbits.

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thegreenrobby t1_j50wgn3 wrote

Agreed. Reducing the complexity of the human experience to "medicine" was a bit reductive on my part, although it certainly plays a part.

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docroberts t1_j510f46 wrote

Fertility is one of many factors in the evolutionary equasion. In evolutionary time human populations have been very scattered At the center of a slightly isolated population this lineage reproduces normally. On the periphery of the isolated population there are more miscarriages, but adequate reproduction for introgression of useful genes into the population. It's probable Neandertal/Sapiens hybrids and Denisovan/Sapiens hybrids were significantly less fertile, yet their genes made it into our pool. Surprisingly the ancestral trees of individual genes are often very different than the species tree.

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off_the_cuff_mandate t1_j513a05 wrote

If the 44 chromosome people survive though, it would likely be without procreating with 46 chromosome people, which would cause them to gradually adapt differently from the 46 chromosome people and eventually become a separate species.

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Slashy1Slashy1 t1_j51f42i wrote

But that requires a substantial population of 44 chromosome people to already exist, at least enough to avoid extreme inbreeding depression.

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harbourwall t1_j52kumd wrote

Inbreeding becomes less dangerous the more common it is, and people in early tribal groups were a lot more closely related than today. Genetic differences between groups increases and all it takes is a bottleneck event to make the tribe of 44s the new standard number of chromosomes. Speciation through increased diversity between many groups of genetically similar individual, followed by selective or random culling of many of those groups.

That's a viable explanation of how we ended up with 46 instead of the 48 the other great apes have.

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Claycrusher1 t1_j50jd7y wrote

Why would 45 chromosomes be a problem but not 44?

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wheatgrass_feetgrass t1_j50jzld wrote

When you reproduce, your chromosomes are split in half. One gamete will end up with 22 and one with 23. This will only create a viable gamete if the one with 22 includes the extra long boy but more importantly, that the set with 23 doesn't include the extra long boy. Duplicate genes aren't any better. Most trisomy conditions are incompatible with life.

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suvlub t1_j50la00 wrote

A sex cell has 1 copy of each chromosome. They are created by meiosis, i.e. a classical cell with 2 copies of each splits into 2 sex cells. During this process, each chromosome finds its buddy, so they split nicely and you end up with 1 copy of each, not random half. That would be bad.

In the person with 45 chromosomes (assuming this specific kind of mutation where 1 chromosome is fusion of 2), the combined chromosome pairs up with random one of the smaller ones, and the other is left without buddy. That's bad. If you are lucky, it ends up in the same cell as the other small chromosome. If you are not, it ends up in the other cell.

The article I linked in the first comment has nice pictures illustrating this.

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yerfukkinbaws t1_j513zuo wrote

The two unfused chromosomes from the parent with 46 will usually both pair with the fused chromosome that came from the parent with 44. This is called a trivalent, instead of the usual bivalent that forms in meiosis. The pairing actually usually goes just fine since the genetic content is not changed and these chhromosome fusions usually involve chromosomes that only had one arm before (acrosomes). What this means is that it's not as random as all that. There is still a chance that separating the chromosomes can go wrong, but the offspring of people who've had a fusion of this type are usually not infertile, just reduced fertility sometimes. Often not even by much and many, many cases are believed to be undiagnosed since there's no "symptoms."

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stickymaplesyrup t1_j50nlvg wrote

Okay, hear me out.

One of the ways we define whether or not species are different is if they can reproduce together, and if the offspring are also able to reproduce. This is why horses and donkeys are still different species even though they can have babies, ie mules, because mules are sterile.

What if these 44 chromosome people grew in number and could have kids together (non-incestuously, I don't know if there are multiple families with this condition)? And then those kids could have kids, and so on.

Would this be the origin of a new species of human?

It's fun to think about and consider.

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ohheyitslaila t1_j515r2u wrote

You actually picked a really interesting set of animals for your example!!! I apologize if I explain anything poorly, I just know a little about this stuff because my family breeds horses. I feel like this kind of hits on what you’re asking.

So female horses and male donkeys can be bred to each other, that produces mules. Female mules have 63 chromosomes, which can’t usually be split evenly to produce a fertile egg. BUT, some female mules actually produce an egg that does have an even set of chromosomes. It’s just that the egg rarely meets up with a sperm with a matching set of chromosomes. A case of this incredible, one in a million chance did occur and a female mule gave birth to a male foal in 2007. The foal had some deformities, believed to be caused by the chromosomal issues, specifically a problem with its legs. But it lived until about 2010, when it slipped on ice and was badly injured, leading to him being humanely euthanized.

I wonder if that kind of thing could happen in humans. It would probably be just as rare, if not more so.

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czyivn t1_j51pj1j wrote

Which perfectly explains why different species frequently have different numbers of chromosomes: It's part of how you get a new species.

Imagine a family of these 44 chromosome people dropped on a desert island with another family of 46 chromosome people. Breeding within a chromosome number group is likely to be more successful than outbreeding. Therefore, over time, a couple possibilities are likely.

  1. The two groups stop interbreeding much and instead carry on as two indepdendent groups which accumulate more independent mutations over time until they are completely infertile with each other.
  2. One of the two groups dies out.
  3. They heavily interbreed. This might result in both groups dying out if there aren't enough fertile individuals in successive generations.

Several scenarios could result, over time, with the emergence of a new population that's not interfertile with 46 chromosome humans. A new species.

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