How do we know everyone today is homo sapiens?

I had a thought today while asked a discussion question on an online course called the History of Humankind. I thought of such an answer to this question that I was thrilled and amazed that that thought came out of my mind. Then I expanded on the answer providing a whole new, plausible way to think about evolution.

The question stated:

Stupid Question: How do we know everyone today is homo sapiens?

I thought about it for a while, my first response being sensible based off scientific data and current theories I thought, "Because we are all here, different species can't reproduce and produce fertile offspring. Only animals of the same species can." Like mules. But then I stopped myself. I thought about new DNA evidence that proves some of us are part Neanderthal. http://news.discovery.com/human/genetics-neanderthal-110718.htm. That led me to an even more important question...

What if we are interbreeding with the species that will eventually out-evolutionize us and we don't know it?

This may sound stupid also, but after thinking further into it, I proceeded. "What is a species but an animal that has evolved in a different climate/environment, one bird has only slight differences from one species to another, including beak size and wing length. Human-made measurements. Us humans have some differences too from apes. Less hair, walk upright, bigger brains. And all of these things evolved over time through our earliest ancestors. Our ancestors we call a different species. But really, you wouldn't recognize one from the other. Think about it this way. 

When no knowledge was available to the world and we all lived in close tribes on Africa, people were able to speak and they spoke of stories and ideas through traveling and interbreeding with other kinds of "peoples". These other peoples could have been hairy or not, intelligent or not, have bigger or smaller brow ridges.  Very slowly, the more intelligent were chosen over the less intelligent, smaller brow ridges got more mates than bigger brow ridges. Over time, they perpetuated these traits until their evidence today. In turn, as homo sapiens got smarter they began to influence their own environment. Taking control they left Africa in search for new environments. This is seen in our earliest ancestors from the mitochodrial Haplogroup L0 who lived about 150,000-170,000 years ago. As well, the Y-chromosome earliest paternal ancestor lived around 120,000-152,000 years ago. The San bushman have a large percentage of these genes.

Check out this Wikipedia site for more information: http://en.wikipedia.org/wiki/Haplogroup_L0_(mtDNA)

Also check out this article: http://www.wired.co.uk/news/archive/2012-09/24/earliest-human-genetic-split

As this haplogroup expanded northward, they became diverse and different haplogroups began to appear. Modern humans moved out of Africa about 125,000-60,000 years ago.

When they met Homo Neanderthalensis who were situated around the Middle East were almost just as intelligent, ate both plants and meat, had funerary rituals, made art (jewlery), and are even found to have the same gene for speech. Our earliest ancestors interacted with them, exchanging ideas and products. Kind of like Columbus did when he met the Indians. They didn't have anything like guns back then so they probably lived in relative harmony with probably a few retaliations. Neanderthals were a different species and looked nothing like our ancestors that were evolving out of Africa. Most research now suggests that Neanderthals were light skinned and red haired, due to the colder climate they lived in, much like the bottom two pictures.

After living in harmony for some time, they were bound to interbreed. This was likely around 80,000-50,000 years ago. Don't get me wrong, Neanderthals looked very similar to modern humans. Because of this, humans did not recognize them as a different species. But they did have some differences. Their bones were much thicker and brow ridges more prominent, traits that adapted them to the cold ice age weather. In this previous ice age, modern humans didn't feel much change as they were in Africa. As the climate changed, Homo sapiens most likely moved out of Africa because it was getting hotter and water was drying up, plants were becoming more scarce. So for their survival, smart Homo sapiens began to move upward to the Middle East. The Middle East's temperature was just right! Smart neanderthals living in northern climates moved southward towards warmer climates. As the climate got warmer, bone density decreased, people evolved to be taller and less stocky. Since these adapted traits would work better evolutionarily, it was those individuals with these traits that were found to be more attractive and mated more. True Neanderthals would have a hard time in the environment and would die earlier in life. Homo sapiens evolved to be roughly a mix of...

 AND

The predecessors of this hybrid spread all over the world, moving to Australia  Europe, Northern Asia, and Southern Asia. In Europe, their skin color lightened. In places closer to equator, India and Spain and Italy, their skin color turned an light brown or olive. Africans are the only race without Neanderthal genetics, which means that no hybrids ventured back down into hot Africa. They only ventured north and east into uncharted land. Land that glaciers were melting from and was providing all sorts of new animals and vegetation. Other species lived with the Neanderthals, Homo sapiens, and their hybrid children...and that were the Denisovans. Evidence shows their mixing with early humans as well, http://io9.com/5715516/another-humanoid-species-co+existed-with-early-humans-and-neanderthals.

Eventually people moved into new lands, unknowingly following the climate change as the animals and plants increased in frequency more northward. Even if each generation one by one moved a couple feet northward, they would have covered the earth by 30,000 years ago. 

Earliest human ancestors in India are thought to date to 50,000-60,000 years ago. http://phys.org/news/2013-06-refutes-early-humans-india-prior.html

Earliest human ancestors to arrive in China, new evidence suggest to be between 42,000-39,000 years ago. http://news.bbc.co.uk/2/hi/6518527.stm.

Earliest human ancestors to arrive in North America were about 13,500 years ago. They followed larger animals such as mammoths down into mid North America as the last ice age melted a land bridge through present-day Canada. They used spears with Clovis points to work together to take down large animals. http://www.crystalinks.com/clovis.html

One type of modern human species, the Cro-magnon (50,000-10,000 years ago) displayed a mix of archaic and modern human traits. This video will show some Cro-magnon men and how similar they appeared to the everyday human you or I would see on the street:

So, getting back to my original question and answer. As you can see, different "species" (as we define them) are separated by only a few tens of thousands of years. We have only been similar to how we appear now for the last 52,000 years or so, maybe less. Of course, as natural selection and evolution play their parts, we will eventually evolve into what we are collectively choosing as mates. Whether we prefer boyfriends who are buff, smart, or with certain skills or looks, it is all playing a part in evolution as a whole. Many people today care much more about someone's intellect or attractiveness than their skin color or brow ridge size. These are traits that will sure be passed to our future species of human. 

To conclude, I would like to take a look at our original question:

How do we know everyone today is homo sapiens?

The answer to this question is that we can't know because...

What if we are interbreeding with the species that will eventually out-evolutionize us and we don't know it?

And the answer to that is...We are.