Thanks for the great question. I will try to answer each part.

Homo sapiens are the only living species of the genus Homo. However, as recently as 40,000 years ago, our species shared the earth with other closely related species of human, notably including the Neanderthals (Homo neanderthalensis).

These two species shared the European continent together for about 5,000 years after modern humans migrated out of Africa. Recent genetic research suggests that there was some interbreeding between humans and Neanderthals. Scientists know this because they compared DNA found in fossilized remains of Neanderthals with DNA from modern humans of European ancestry and found that they share around 5% of the same genetic material.

This means that people of European descent today share some of their genes with another species of human!

Sowhy are the Neanderthals extinct? Genetic and archaeological evidence shows that Neanderthal populations began to dramatically decline as modern humans migrated. It is hypothesized that modern humans brought with them pathogens or germs from Africa that the Neanderthals were not immune to, and disease lead to their extinction.

From what they produced and left behind, we know the Neanderthals made stone tools, used fire, and hunted just as modern humans did around the same time. So, they at least approached modern humans in terms of intelligence. There is also debate as to whether the Neanderthals cared for their sick and buried their dead – if true, this would be evidence of advanced symbolic thought equal to modern humans.

Despite this, Neanderthals were apparently less capable of living together in large groups, and anatomical studies show that their throats were not capable of making any sort of complex language that modern humans are naturally able to produce. Modern humans, then, were able to out-compete all other species of humans, pushing them into extinction.

Thanks for the interesting question,

Most of us have some extinct human-like species in our DNA - our genes. We have up to 3 or 4% Neanderthal DNA in our genes. Neanderthals lived in Europe and Asia when humans came out of Africa.

Here's an article about this: read here

Good question! There were a lot of other extinct species of humans. All are more closely related to us than any other living animal, but some are more closely related to us than others, and many are more closely related to other extinct species than they are to us. A few of them still have some of their DNA in humans: modern humans of European extraction have about 4% of their DNA from Homo neanderthalensis, and there was an Asian species that modern Asians can have up to 10% of their DNA from. Obviously, these species and H. sapiens were partially interbreedable, at least some of the time.

I don't think we know what caused their extinction. Some of it was undoubtedly climate change associated with the coming and going of the ice ages, but there may have been other causes as well. A few of them H. sapiens might have wiped out.

The different species had a wide variety of different tools, so some of them were certainly more intelligent than others. Some, like H. neanderthalensis, may have been as intelligent as H. sapiens, but many of the others were clearly less intelligent. Unfortunately in most cases all we have are their tools, and only their stone tools, so if these extinct human species made sophisticated tools out of wood, for example, we would have no knowledge of it.

Also, the social consequences of having multiple species of humans around would be ugly, to say the least. You've probably heard of racism, where people of different skin colors can hate each-other for no reason other than their race. All of these different "races" belong to one species, H. sapiens, and any intellectual differences between these "races" are so small that they are difficult to measure and may not exist at all. Now imagine what it would be like if there really were more than one species of human in the world, some of which really were more intelligent than others!