The reason

Every year about 3500 people die choking on food, typically mochi or rice dumpling in 正月. Do you know we are the only animal that can get food clogged in the airways? This is an arresting story from Bill Bryson.
Exactly 150 years ago, an archaeologist examining a cave in Neander Valley, found a skull of a type they had never before seen. The skull was from a person belonging to a race of people living all over in Europe till tens of thousands years ago. They decided to call the person Neanderthal(ネアンデルタール人).
Neanderthal man was very different from modern man. He was short, only 150cm tall, robust, with a small forehead, long nose and heavyset features. He had a larger brain than us despite the stupid appearance somewhat resembling the president of that large country. He wore clothes, used tools, engaged in communal activities. He buried the dead and marked the grave with stones, which suggests he may have done some religious ritual. He even looked after handicapped members of his family. All of this suggests he spoke a language.
About 30000 years ago, all of a sudden they disappeared from the globe possibly displaced by people called Cro-Magnongs（クロマニヨン人). Cro-Magnons are taller, more agile, more handsome (by our standards) and identical to us.
Then why could they overwhelm Neandethals? It has very much to do with choking on food.
It's getting a long story already. Please be a little more patient if it's going into somewhat technical.
We, Cro-Magnons somehow have our larynx(声帯) deeper in our throats than that of any other animals' including Neandethals. Air inhaled through nose once enters our esophagi(食道) and then into the windpipe(気管). Food and drink must pass over the larynx and the windpipe lid, which opens and closes everytime we swallow soemthing, on the way to stomach. This causes a risk that some will be inadvertently inhaled. Neanderthals, chimpansees, and all other animals have no contact between their airways and food pipe. They can swallow food and breathe at the same time. By the way human babies do not have their larynx in position right after they are born. They are like other animals by birth. So they can drink milk while breathing from nose. But their larynx descend to the lowered position sometime between the ages of 3 and 5 months, the precise period when they likely to suffer from Sudden Infant Death Syndrome(乳児突然死).
Anyway the point is, at the cost of the choking risk, Cro-Magnons acquired the ability to speak clearer in more sophisticated way. The lowered larynx made a space for the voice to resonate(共鳴) and allow muscles to develop around the larynx to give it a more delicate control. That made a great difference. Being able to speak more complicated things must have made their thought more systematic and organised. They are considered to have linguistic system sufficiently sophisticated to deal with the concept such as "Today we are going to hunt deer. You guys run shouting into the woods to drive the deer out of it while we'll be waiting at the end of the woods to kill them with spears. The game will be shared equally among us men and women." By comparison Neanderthal speech may have been something like "I'm hungry. Let's hunt." because their speech cannot have been clear enough to tell the detail.
Just the slightes evolutional change brought with it a big consequence of survival and extinction. Thus we survived and Neanderthals got annihilated. After all we are a creature of chance, very improbable one. It would be safe to say that I'm writing this thanks to my lowered larynx.
Sorry for the long entry. It is quite hard to make a long story short. 書き出すと止まらなくなる。