When Darwin set foot in Rio de Janerio he witnessed the brutality of slavery. Brazil is a melting pot of genetic diversity. All fossil and genetic evidence points that Africa is the home continent where modern Humans originated about 200 thousand years ago. As Humans spread and colonized new continents genetic diversity eroded gradually proportional to the distance away from Africa. Loss of genetic diversity was because of joint effects of two well understood genetic phenomena known as population bottleneck and founding events.
One of the invited guests is Spencer Wells recognized by the Genographic Project that created a detailed genetic family tree of modern Human populations currently spread to all non-Antarctic landmasses. Geographically South America is one of the farthest locations away from Africa and thus harbors a population that had been bottlenecked multiple times throughout the Human migration (see the map below). First major bottleneck happened when Human ancestors crossed the Beringia into North America. Second major bottleneck happened after crossing of the Central American isthmus to South America. As has been encapsulated marvelously in Guns Germs and Steel by Jared Diamond continents with North-South axis are more difficult to travel due to fast changing environment and climate. Crossing deserts and tropical rainforests can be rather challenging.
This is when the story gets interesting. One of the unintended consequences of slave trade was the injection of African genes into South American populations. Gene flow from Africa enriched Human genetic diversity enormously since Africa is the motherland for all humanity and therefore harbors the richest gene pool accumulated over hundreds of thousand years. Geneticists call this type of restoration of lost genetic variation as genetic rescue and as we see in this episode Brazilian population is mixed with Native American, European and African genetic backgrounds.
Genetic rescue: Restoration of genetic diversity by the introduction of unrelated individuals into a population with reduced genetic diversity.
The episode also covers some fossils discovered by Darwin while he was carrying out a few expeditions overland. Among these fossils are extinct mammals including Taxodon and Megatherium. There’s also an emphasis on modern sea mammals especially whales and orcas. Whales fascinated Darwin for their peculiar land adaptations such as lungs, eyelids, skeletal structure of limbs. Whale evolution is quite striking who retained limb morphology of tetrapod fossils that were the ancestors of all land vertebrates (animals with backbones).
“The white race was thought to be separate and superior to the black race on a series from the primeval worm to the angels. Darwin’s theory released us from such nonsense. Homo sapiens is one undivided species there are no ladders or rungs in the process of the evolution of life.”