If you intend to open a restaurant, you ought to check the local competition. If there is a Chinese restaurant in the neighourhood, you might consider opening an Italian restaurant. Or you could open a fast food eatery in an area overflowing with expensive, high-class restaurants. But if there are too many restaurants in the area, one of them is going to go to the wall.
What you need to understand is that the same applies to biology. A species' way of life is called its niche, and natural selection ensures that individual niches do not overlap too much. There are two species of rhinoceros living side by side in Africa, but one is a grazer and the other is a browser, so they get on well enough. When you watch one of those African documentaries, you should be aware that every species of gazelle, antelope, wildebeest, buffalo, or zebra consumes a slightly different type of food or exploits a slightly different microenvironment. In archipelagos it is not uncommon for two similar species to exist on one island, but only a single species on another. In the latter case, it is normal for the single species to expand its niche to include much of what the other one would have occupied. That is why the biggest leopards in the world live in Sri Lanka; there are no tigers there. If two species occupy the same niche, the less efficient one will go to the wall. Because of this the thylacine became extinct on mainland Australia once the dingo arrived, and why the British red squirrel is retreating before the advance of the introduced grey squirrel.
So what would happen if two species each occupied an extremely broad niche - like intelligent, tool-making generalist, similar to Homo sapiens? Can two intelligent species, particularly unrelated ones, evolve on the same planet? A lot of science fiction writers seem to think so.
But we don't need to ask, because it has already happened. Approximately 400,000 years ago, the human lineage split. Those in African evolved into us, those in Eurasia into the Neanderthals. In the last few years, a brand new species, a branch-off from the Neanderthals, has been discovered in Siberia: the Denisovans, named for the cave in which it was found. It is also likely that a strange group of fossils in the Red Deer Cave of China may have been another species. It is becoming ever more obvious that a number of small twigs of the human family tree once developed into fully-fledged species. But, by and large, the two most successful species, the ones which occupied the broadest territory for the longest period, were Homo sapiens and Homo neanderthalensis.
But now there is only one. They came into contact 60 or 70,000 years ago, when our ancestors left Africa, and by about 28,000 years ago the Neanderthals were extinct. We did not exterminate them. Over those 30 or 40 millennia, the two species would have run the full gamut of interactions, from violence, to avoidance, and to friendship. We even mated with them. It is now established that while black Africans interbred with other archaic humans, the rest of us all possess a few percentages of Neanderthal genes (and Australian Aborigines have Denisovan genes as well). However, the two species occupied the same niche, they were competing for the same resources, and one of them had to go. It has been calculated that a 2% inferiority in breeding success would have put the Neanderthals on a downward spiral to extinction.
In popular culture the Neanderthals have been visualised as dull, brutish first attempts by nature at her program of evolving her pièce de résistance: us. This is not true. The Neanderthals were as human as us, but a different type of humanity. They had bigger brains than ours. Estimating their IQ is impossible, but it is likely that any deficiency in comparison to Homo sapiens would be in an ability the IQ tests cannot measure: creativity. Left to themselves, they would probably in time have discovered agriculture and developed civilisation. They did, however, possess two characteristics of our ape ancestors which we have lost. Firstly, they were incredibly strong. Secondly, they grew and aged faster - more like a chimpanzee than a modern human. A five-year-old child looked like an eight-year-old modern human, a 40-year-old was as decrepit as a 70-year-old. They could have outbred us. On the other hand, each generation had less time to learn and to pass on its skills and invent new ones. When the crunch came, this may have been where our advantage lay.
So there you have it: two major and one or two minor intelligent species evolved on the same planet, and all but one became extinct while they were all in the hunter gatherer stage. It is possible to imagine other scenarios. Suppose the Denisovans got to Australia first, and then the continent became, by some geological quirk, completely isolated until modern times. Then, when the first European settlers arrived in the eighteenth century, they would have found it occupied, not by Stone Age Homo sapiens, but by some mysterious separate species of humanity, also in the Stone Age. (Agriculture and of civilisation require certain preconditions such that, although they can arise more than once, they will do so on different time scales. Added to this the different abilities and intelligence of different species, and it is next to impossible that two species will achieve technological equivalence simultaneously.)
Africa and Ice Age Eurasia were poor ground for the development of agriculture, which commenced first in the post-glacial Middle East. But suppose Africa and Eurasia were somehow isolated at the end of the Ice Age, such that the Neanderthals discovered agriculture first. Would they have eventually moved into Africa, and outcompeted our species, leading to our extinction - or perhaps leaving a remnant population of Homo sapiens in the jungles of West Africa, whose horrible diseases would keep out the Neanderthals as they once made it the "white man's grave"? Or is our species really superior under all conditions, such that once we had learned agriculture from the Neanderthals, we still managed to push them to extinction?
These scenarios are about the best a realistic science fiction writer can offer. But modern humans and Neanderthals were closely related. What about the evolution of two unrelated intelligent species? Suppose the Americas were completely isolated from Eurasia, so that when Columbus finally arrived, he found, not Indians, but a second human-equivalent which had evolved from the New World monkeys? Forget it! That would imply that two parallel streams of evolution advanced at the same rate. Refer back to my article on geological and historical time. The evolution of intelligent life forms is probably inevitable once life begins, but it still requires specific conditions to ignite it, and the chances of it happening simultaneously twice on different continents are so remote they are effectively non-existent.
No! You may rest assured that when we finally discover intelligent life in outer space, it will have evolved just once per planet.
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