When humans other than sapiens walked the earth

There are 3 elephant species, 10 of pigs, 90 of whales and 260 species of monkeys on earth. All humans today, no matter which colour, height or region, constitute just a single species, homo sapiens. If man is an animal (as he demonstrably is), it begs the question as to where are all its different species ? The answer, it turns out, is quite disturbing.

Around 1.8 million years, when there wasn't even a whiff of any sapiens on earth, a brute, with a little over half our brain-size and a radically different anatomy, emerged in the African grasslands. Armed with improved intelligence and full bipedality (walking on two legs), it became the first human to cross the continent of Africa. In less than a million years, it had colonized lands as far as China and Indonesia, mastering fire in the process. This was homo erectus - the most successful human species to have ever lived, at least in terms of time (over a million and a half years). 

Around 600,000 years ago, a group from an extinct human species called homo heidelbergensis migrated out of Africa, choosing to settle in what is now Europe. This group evolved into the famous Neantherthals - a culturally-sophisticated species that surpassed modern humans in both physical strength and brain size. It dominated the European and West Asian landscapes for a whopping 300,000 years! (let that figure sink in - all the history we learn spans not more than 5,000 years)

As fate would have it, another group of the same homo heidelbergensis species had stayed back in Africa. In another 400,000 years, these relatively non-descript cousins of the illustrious Neanderthals would evolve into the most dangerous predator the earth had ever seen - us, the homo sapiens. ¹

Evidence indicates that we did try to cross Africa about 100,000 years ago, but were driven back by the Neanderthals while passing through the Middle East ². Close on the heels of that failure, the largest supervolcano eruption of the last 25 million years occurred in Indonesia, setting off a thousand-year winter. Our species was pushed to the brink of extinction ³, with genetic evidence revealing that not more than 10-15,000 people survived. Evolution worked overtime, and although nobody knows exactly what cognitive changes occurred during this time, the next time we invaded the Middle East 70,000 years ago, we were a completely different animal altogether. 

Within a span of five thousand years, we obliterated the Neanderthals (and possibly the Denisovans - a human species that we know only through its DNA), proceeding then towards South Asia to wipe out any and all surviving descendants of homo erectus. 

Around 40,000 years, we became the first human species to set foot on Australia - a truly devastating moment in the continent's history. Almost its entire megafauna (a name given to large-sized animals) was slaughtered out of existence. In order to obtain easy prey and clear forests, we literally set fire to the entire continent! Eucalyptus - the most widespread tree in Australia today, survived only because it was fire-resistant. The pattern was repeated everywhere this new species of humans went - in North America, 34 out of 47, and in South America, 50 out of 60 genera (entire genera, not just species) of large animals disappeared within a couple of thousand of years of human arrival. So much for the belief that our ancestors lived in harmony with nature!

So again, where are all our human cousins ? In oblivion of course, where we placed them. But before dying, they did manage to leave an indelible mark upon us. While contriving to end their existence, we had duly taken care to breed with some Neanderthals, Denisovans and a third yet-unknown species, resulting in 1-5% genetic contribution from them in modern non-African populations. In a very real sense therefore, a part of these ancient human species continues to live within us.

References : 

1. Sapiens : A Brief History of Humankind
2. The Greatest Show on Earth
3. Human Evolution : Past and Future (University of Wisconsin)
4. Introduction to Human Evolution  (Wellesley College)

¹ Its possible that the descent from homo heidelbergensis wasn't entirely linear
² There were probably other minor migrations, but firm evidence for them is still pending 
³ Whether it was the result of this supervolcano eruption or some other  unknown cause, remains a contentious issue

(Image courtesy of africa at FreeDigitalPhotos.net)