It is probably trite to question whether they could have but we must also enquire whether they would have. Stating the obvious, they could have subject to the caveat that they only would have if they were members of the same species as different specious refuse to mate.
How and why did these other species of humans disappear? The answer to the “same species” question will determine which theory takes primacy: Interbreeding or Replacement?
Main picture: Theoretically the DNA of the Neanderthals & Homo Sapiens should be so far apart that they never should be able to interbreed. However is that correct?
Dispelling a myth
First some biological nomenclature. The primary determinate of whether two different looking animals can procreate, producing offspring which itself can produce offspring is not whether they look the same but rather whether they are of the same species.
Because of this fundamental misunderstanding, the absurd notion arose more than a century ago that humans of different races should not procreate. This was known as miscegenation. If that were true, then my German Shepherd dog and Maltese cannot procreate either. Yet they can. Similarly so can people of different tribes and races. Technically, they can due to the fact that they belong to the same species.
In the case where the different animals cannot procreate irrespective of whether they look similar such as leopards and cheetahs, they belong to same genus but they are different species. In other words the genus cat comprises many species which are unable to procreate. As all household cats like dogs, or humans for that matter, belong to one species, hence have no difficulty breeding.
What is known is that one segment of a species can gradually evolve to the point where they are no longer able to breed with former members of that species. At that point, there will be two species instead of one. Generally, the factor which facilitates this process is lack of contact over a prolonged period. The horse and the donkey are an excellent example. Their genetic material is now so diverse that they cannot produce offspring which can breed. The offspring – a mule – is itself incapable of breeding. Given another extended period, even their ability to reproduce will no longer be possible.
Were there multiple human species in the past?
What is known is that evolution does not occur in a straight line. The initial exodus of proto Homo Sapiens from Africa occurred over two million years ago. These members of the genus Homo evolved over the millennia into different species as they dispersed around the world. Europe had its Neanderthals, Russia had its Denisovans and the Indonesian island of Java had its Homo Solonsia. On another Indonesian island, homo evolved into the diminutive Homo Floresiensis which reached a maximum height of one metre and a weight of 25 kilograms.
About 100,000 years ago, another diaspora occurred. At that moment in history, the world comprised of at least half a dozen hominid species with Homo Sapiens being only one. Imagine if that situation had still prevailed. Would these different species have been viewed as a threat or more demeaningly treated as if they were apes, gorillas and chimpanzees and locked in cages?
Knowing man’s predilection of treating one’s inferiors with disdain, this is highly likely.
Latest genetic evidence
Until the past decade, the prevailing view is that as these types of hominids were sufficiently different to be classified as different species. Because of this, the Replacement Theory has been bestowed with the privilege of being victorious.
Amongst the reasons why the Replacement Theory has been given undue prominence is the risk of opening a can of worms. No scientist relished the possibility of the political dynamite detonating if the converse was proved to be valid. It would raise the spectre of explosive racial theories being advanced.
That was until five years ago.
After a four year exercise to map the Neanderthal DNA, in 2010, the results were announced. The scientific community was stunned. Of the current populations in Europe and the Middle East, one to four percent of their DNA comprised that of Neanderthals. Even more surprising is the DNA extracted from a finger of a Denisovan skeleton. This provided proof that 6% of its DNA is now found in Homo Sapiens’ DNA.
Fortunately the quantities are so low that it does not invalidate the Replacement Theory as being the prime reason for the disappearance of the other hominid species. What it also indicates is that these species were almost unique species and hence, barely able to interbreed
Further tests are being performed. These will confirm or invalidate these initial numbers. Fortunately, the percentages are immaterial. If not, mankind would have been woefully unprepared for a possible racist debate.
A growing consensus since 2010 readily accepts the reality of marginal genetic differences as being a reality.
Sapiens: A Brief History of Humankind by Yuval Noah Harari