EVOLUTION (11:704-486) – TROPICAL
The first humans entered South America about 15,000 years ago, as nearly as we can
tell, across the Isthmus of Panama. In their own way, they were just as new as the ancestors of
Darwin's Finches in the Galapagos. There were probably not large numbers of them at first, and
they brought a subset of the genetic variation available from NE Asia (
- They must have spread rapidly in this virgin habitat, because we have
archaeological sites from the Andes and the Guyana Shield, dating to about 14,000 years BP.
By 6-10,000 BP, they had reached Tierra del Fuego, at the southern end of the continent.
It wasn't until about 5,000 - 6,000 BP that they occupied what is now the tropical rainforest
of lowland South America. I say that advisedly, because the tropical rainforest is not ideal
zone for archaeological remains. Everything is biodegradable; nothing much is preserved.
There are sites in Central America, dating to 10,000 BP, so their absence from lowland S.
America is probably telling us that humans hadn't really occupied the zone. I have to say that
this is tough habitat for humans, and something major had to happen before occupation.
Nevertheless, people did eventually occupy this zone, and it is those people I want to talk
about today. Even by the 1980s, you could fly for hundreds of miles, and all you'd see were
tropical forest and rivers. There would be people, but you wouldn't see them (
Slide - 02
– Occupation required the evolution of agricultural practice for tropical rainforest.
Corn was domesticated in Mexico and/or the Andes, only about 7,500 years BP, and other
crops aren't much older. To sustain a meaningful population of humans, agriculture is a must.
The difficulty is that soils in the tropical rainforest are very limited in nutrients. When you
look at a tropical forest, it is easy to be fairly impressed with its richness. It is bursting with
life. But all of the nutrients in the system are tied up in the vegetation, mostly the leaves, and
in the top 3-4 inches of the soil profile, that is to say, in the leaf litter on the forest floor.
To have agriculture, you have to clear the overstory and the shrubs. You need to cut the
forest and burn the slash, before you can plant. When you do that, you have dissipated most
of the nutrients in the system. You get the nutrients right at the surface, but then it's over.
Tropical temperature is so equable and moisture so abundant that biodegradation is almost
explosive. The fungi, bacteria and small critters in the soil do a number on the leaf litter,
which doesn't last for long. The only reason it lasted in the forest was that there was a steady
and prolific rain of detritus from above, continually feeding the soil biota, down below.