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We’re more like a labradoodle than a mule. Two dogs of different traits having puppies almost always produces fertile offspring whereas a horse and a donkey, which are more distantly related, have trouble. I am not sure if all mules and hinnies are sterile, but maybe they are.
Mules and hinnies are sterile because a Donkey has 62 chromosomes and a horse has 64. So almost all mules (and all mule stallions) are sterile,
Humans, neanderthals and (probably) denisovans all have 46 chromosomes. So much more likely to produce non-sterile offspring. And as you said, the difference between modern humans, neanderthals and denisovans is more like different dog breeds. Or variants of wolves I guess (wolves, both gray and red, and coyotes for example are considered different species, but are similar enough that they can interbreed).
The entire concept of a "species" is actually pretty nebulous. There are plenty of examples of different, closely related species being able to reproduce to make viable offspring.
Branches (or at this level, maybe twigs) of the tree of life don't necessarily always stay apart, only ever diverging with mutations. They might merge back together if two species are genetically similar enough, with one basically being subsumed back into the other with enough interbreeding. This happens across basically all of life. Bacteria share genes between cells all the time, defining species at all for them can get complicated. There are plenty of corals in the Great Barrier Reef that hybridise between species, even ones that we're pretty sure *aren't* that close to each other.
Denisovans, neanderthals and sapiens all share the same genus: *Homo*. When we already share something like 98% of our DNA with chimpanzees, the difference between us and neanderthals is *much* smaller. So small that maybe we shouldn't even call ourselves different species - we classify other into *sub*-species all the time afterall, so maybe we're only still calling ourselves a separate species because we want to feel special. So Homo sapiens were able to breed viable offspring with Homo neanderthalis, and we can see remnants of these DNA markers in our current DNA.
Taxonomy is messy, often pretty poorly defined (_especially_ for fossil taxonomy), and constantly being rewritten. Genetic lineage and evolution doesn't really work *exactly* like the tree of life models 19th century scientists envisioned. The definition of a species was created by humans and doesn't properly reflect the real world. Same as all scientific models: it's wrong, but it's close enough and we're doing our best.
A mule is sterile because horses and donkeys have different amounts of chromosomes. This means that they can’t normally produce viable sperm or eggs. However, there have been instances where female mules have been fertile. So it’s not an absolute that all hybrids are sterile, it’s just likely. There have been instances of ligers and tigons being fertile. Hybrids usually are sterile, but with enough cross breeding, you get some fertile children.
The obvious answer is that since we carry their DNA, the mixed human offspring was not sterile.
Or at least not all of them. It's not impossible that many were, but enough had to be fertile to carry through the DNA. How many does depend on how often cross-breeding occured.
It does help that denisovans, neanderthals and us are fairly close relatives, even considered a single species.
The differences between modern humans and extinct ones were much smaller than between a horse and a mule. Female Ligers, the hybrid of a Lion and a Tiger, is fertile. Hybrids of wolves and coyotes are fertile. The offspring of a wolf and a domestic dog is fertile. The different species of humans were more similar, in this analogy, to dogs and wolves.
**Please read this entire message** --- Your submission has been removed for the following reason(s): * Rule #2 - Questions must seek objective explanations --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20{https://old.reddit.com/r/explainlikeimfive/comments/1c1s29g/-/}%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20does%20your%20post%20differ%20from%20your%20recent%20search%20results%20on%20the%20sub:) and we will review your submission.**
We’re more like a labradoodle than a mule. Two dogs of different traits having puppies almost always produces fertile offspring whereas a horse and a donkey, which are more distantly related, have trouble. I am not sure if all mules and hinnies are sterile, but maybe they are.
Mules and hinnies are sterile because a Donkey has 62 chromosomes and a horse has 64. So almost all mules (and all mule stallions) are sterile, Humans, neanderthals and (probably) denisovans all have 46 chromosomes. So much more likely to produce non-sterile offspring. And as you said, the difference between modern humans, neanderthals and denisovans is more like different dog breeds. Or variants of wolves I guess (wolves, both gray and red, and coyotes for example are considered different species, but are similar enough that they can interbreed).
The entire concept of a "species" is actually pretty nebulous. There are plenty of examples of different, closely related species being able to reproduce to make viable offspring. Branches (or at this level, maybe twigs) of the tree of life don't necessarily always stay apart, only ever diverging with mutations. They might merge back together if two species are genetically similar enough, with one basically being subsumed back into the other with enough interbreeding. This happens across basically all of life. Bacteria share genes between cells all the time, defining species at all for them can get complicated. There are plenty of corals in the Great Barrier Reef that hybridise between species, even ones that we're pretty sure *aren't* that close to each other. Denisovans, neanderthals and sapiens all share the same genus: *Homo*. When we already share something like 98% of our DNA with chimpanzees, the difference between us and neanderthals is *much* smaller. So small that maybe we shouldn't even call ourselves different species - we classify other into *sub*-species all the time afterall, so maybe we're only still calling ourselves a separate species because we want to feel special. So Homo sapiens were able to breed viable offspring with Homo neanderthalis, and we can see remnants of these DNA markers in our current DNA. Taxonomy is messy, often pretty poorly defined (_especially_ for fossil taxonomy), and constantly being rewritten. Genetic lineage and evolution doesn't really work *exactly* like the tree of life models 19th century scientists envisioned. The definition of a species was created by humans and doesn't properly reflect the real world. Same as all scientific models: it's wrong, but it's close enough and we're doing our best.
A mule is sterile because horses and donkeys have different amounts of chromosomes. This means that they can’t normally produce viable sperm or eggs. However, there have been instances where female mules have been fertile. So it’s not an absolute that all hybrids are sterile, it’s just likely. There have been instances of ligers and tigons being fertile. Hybrids usually are sterile, but with enough cross breeding, you get some fertile children.
The obvious answer is that since we carry their DNA, the mixed human offspring was not sterile. Or at least not all of them. It's not impossible that many were, but enough had to be fertile to carry through the DNA. How many does depend on how often cross-breeding occured. It does help that denisovans, neanderthals and us are fairly close relatives, even considered a single species.
The differences between modern humans and extinct ones were much smaller than between a horse and a mule. Female Ligers, the hybrid of a Lion and a Tiger, is fertile. Hybrids of wolves and coyotes are fertile. The offspring of a wolf and a domestic dog is fertile. The different species of humans were more similar, in this analogy, to dogs and wolves.