For the better part of the Late Pleistocene (which began around 130,000 years ago), there were three species of hominid treading this dusty Earth: Homo neanderthalensis, Homo floresiensis and Homo sapiens. By the Holocene (around 10,000 years ago), there was only one — H. sapiens. Researchers and academics have long debated the cause of this winnowing-out of the genus Homo, and the hypothesis de jure is the so-called 'Upper Paleolithic Revolution', put forth by Richard Klein of Stanford University. Basically, this hypothesis seeks to explain the appearance of evidence for behavioral modernity in anatomically-modern H. sapiens in the archaeological and fossil record, and proposes that this shows evidence for a great advance in human cognition that allowed them a selective advantage over the other two hominid species co-existent with humanity at the time.
According to this hypothesis, the 'Bang' took place approximately 50,000 years ago, although fully anatomically-modern humans have been found as far back as 130,000 years ago in Africa. With this 'Great Leap Forward' (or 'Cognitive Big Bang', as it's sometimes referred to in popular literature), we see a number of new things suddenly appearing in the archaeological record: ornamental jewelery, sophisticated weapons, art, ritual artifacts and finely-made tools. Basically, modern human behavior just popped up 50,000 years ago, and shortly thereafter, we became the only ones in the genus Homo still around.
Prior to this event, the state of the art in hominid tool-making was the Mousterian tradition — a prepared-core method that could produce some pretty decent handaxes, points and scrapers. This remained the cutting-edge (excuse the pun) for tens of thousands of years with little improvement or modification. The genus Homo remained more-or-less in technological stasis.
But after the Bang, the human species went through four or five different traditions, all within the space of less than 30,000 years! Innovation is evident throughout the archaeological record and we see an increase in the number of projectile points and blades made. It is here that we also see the appearance of the atlatl and later, the bow and arrow. H. sapiens was now able to easily kill at a distance, something that the Neandertals and the Flores islanders could not do. Effective projectile weapons might well have proven an incredible (and insurmountable) tactical advantage, if indeed H. sapiens were directly responsible for the extinction of H. neanderthalensis and H. floresiensis. On this subject, the paleoarchaeological record remains silent.
But you needn't kill a species off directly to cause its extinction, and all the weaponry in the world can't guarantee you another millennium or two of survival. Post-Bang, H. sapiens started getting creative. It is after this event that we see cave paintings, commonplace use of grave goods, bodily ornamentation and sculpture. H. neanderthalensis also exhibited these behaviors, but many claim that they were simply imitating us H. sapiens. I personally beg to differ, but only because I believe that Neandertals and anatomically-modern human beings simply interbred, and the superior numbers of H. sapiens simply drowned out the Neandertal contribution to the human genome. H. neanderthalensis was geographically very restricted in range, and there're some intriguing pieces of fossil evidence that show 'hybrids' between H. neanderthalensis and H. sapiens (see: the Skhul cranium and the Lagar Velho child).
The problems with this hypothesis are as follows:
- If there were an advancement in the cognitive capacities of H. sapiens, nothing shows in the fossil record. Our heads didn't get bigger, though our jaws and dentition were getting progressively smaller as a result of increased use of tools in place of teeth. Of course, a brain needn't be bigger in order to be more efficient; this advance may well have been a reshuffling of the cerebral architecture of H. sapiens, and soft tissue (like brains) doesn't show up in the fossil record at all.
- Absence of evidence isn't evidence of absence. Perhaps Neandertals did make cave paintings and bodily ornamentation; but we'll likely never know, because the archaeological record is extremely spotty. To illustrate: all the fossil evidence for the genus Homo will fit in the bed of a pickup truck. When something does make it to the present day in the archaeological record, we have a series of very fortuitous conditions to thank for it.
- This smacks of sapiens chauvinism — the notion that there's something inherent to H. sapiens that makes us better than everything else on this planet. Neandertals actually had bigger brains than we did (though they also had more robust bodies, so it balances out in terms of encephalization ratios). Neandertals weren't nasty, sluggish brutes — they were damned clever, and we simply don't know how they disappeared. Some imagine they got killed off by H. sapiens, and I personally wouldn't put it past us. But it seems more likely (to me) that we simply merged; we might well have been closely-related enough to interbreed. Until the Neandertal Genome Project is completely analyzed and debated over, we won't know for sure.
- Likewise, just because archaeological evidence popped up 50,000 years ago, pointing to the possibility that H. sapiens just got a whole lot more creative, doesn't mean that it happened at that point. For all we know, we could have been making cave paintings and atlatls long before then. But because the archaeological record is very hit-or-miss, we have to go with what we got as evidence, while reminding ourselves that there will always be the possibility of further evidence that forces us to modify (or completely throw out) our hypotheses.
This is an interesting hypothesis, but as in all science, it must be adapted to new evidence. It may very well be that H. sapiens fought fist-and-spear with H. neanderthalensis and hunted down H. floresiensis mercilessly in the forests of Flores. It may very well also be that we simply out-competed them and overran them with our superior numbers, pushing them to ever-more marginal environments, as agricultural societies did to foraging societies in more recent times. Or perhaps we made love, not war, and washed away most of the genetic traces of human-Neandertal interbreeding with our superior numbers, aided by the fact that the Neandertal and human genome have more than 99.5% commonality between each other. Who knows? Until we invent a time machine, we'll never know for sure. Until then, all we can do is look at what little traces remain of the people that came before us, and make educated guesses on what we find.