Given that there were other human species (homo this and homo that - homo, or human, is our genus, not our species), there was interbreeding between them. This has been proven via analysis of DNA.
Most famously, just within about the last 15 years, Neanderthal (homo neanderthalis) DNA was sequenced and compared to modern humans, and it was discovered that Eurasians (Europeans, Asians, and indigenous Americans) have around 2% - 4% Neanderthal DNA.
I've recently discovered that some Afrocentrists like to use this fact to classify white people as "cave men," and suggest we're not fully human. It's unclear whether they realize they are also implicitly classifying other people of color as less than fully human, too.
But genetic evidence shows that Africans' ancestors also interbred with other human species. We haven't been able to attach that genetic code to any specific other species yet, but it's clear in people's DNA. So, umm, sorry, Afrocentrists, your ancestors got frisky with non-homo sapiens, too.
But that doesn't mean anyone is less than fully human. Those other homo species were fully human, too, just not homo sapiens.
Clearly they were closely related enough to produce some fertile offspring. How close is not yet known. We do know that there are other species that sometimes crossbreed (although in the wild it can be rare).
For example, mules are the offspring of a male donkey and a female horse (a hinny is the offspring of a female donkey and a male horse). Because donkeys and horses don't have the same number of chromosomes, mules are usually sterile. But there are (very) rarte exceptions (and apparently only when a female mule is mated with a horse or donkey, not another mule).
A cross-species mix that is reportedly becoming common in the US is the coydog, or even the coydogwolf. Coyotes, wolves, and dogs are all closely related enough that they can mate and produce fertile offspring. I'm not sure about whether there are any male/female constraints here (other than perhaps large pup size from a wolf sire potentially being deadly for a smaller dog or coyote to birth).
There are a few reports of pizzly bears - the offspring of a polar bear and a grizzly.
And then there are ligers (lion father, tiger mother) and tigons (tiger father, lion mother). In both animals, males are reportedly sterile, but females frequently fertile.
So with multiple human species roaming around in the same territory, it's not really surprising that our ancestors got it on with them at times, perhaps both consensually and non-consensually. Those other species are all extinct now, but their legacy lives on in the last remaining human species.