Essay:Human Evolution (short version)
It is disturbing to discover in oneself these curious revelations of the validity of the Darwinian theory. If it is true that we have sprung from the ape, there are occasions when my own spring appears not to have been very far.—Cornelia Otis Skinner, The Ape in Me
200,000 years ago (give or take some tens of thousands), one member in one of the tribes of hominids in Africa was born with a genetic mutation that made him or her look like us. This tribe and perhaps other tribes found this mutation attractive, and soon (over some thousands of years) the descendents of this individual dominated the forests and plains of Africa.
From time to time, the hominids who looked like us went out of Africa (as hominids had been doing for hundreds of thousands of years). Those who left Africa did not thrive for long. Catastrophes such as volcanic eruptions and glacial periods doomed most of the hominid tribes of Europe and Asia.
Between 80,000 and 70,000 years ago (more or less), during a glacial period in the current Ice Age, volcanic ash darkened Earth for at least six years (the Toba Catastrophe). During this time, the harsh climate annihilated most of the hominid tribes not only in Europe and Asia but also in Africa. In Africa, our ancestors may have been reduced to fewer than ten thousand individuals. Genetic testing shows that modern humans are descended from those thirsty and shivering survivors in Africa. 70,000 to 60,000 years ago, some of the descendents of those survivors decided to explore Europe and Asia once again. Thereby, those descendents took dominion over the world.
The small population which lived 70,000 to 60,000 years ago may have survived either because of happenstance or because that population had a greater average intelligence than the tribes which had gone before. If that population survived merely by happenstance, then it is unlikely we are smarter than our ancestors of 200,000 years ago. If that small population survived because it was smarter than other populations, then our species evolved, that is, we are the inheritors of genes which make us more intelligent on average than our ancestors of 200,000 years ago.