Migration has, throughout human prehistory, both spurred and slowed human evolution.
Prehistoric humans were hunter-gatherers. Dependent on the local flora for something approaching 70% of their total calories, they hunted for the remaining 30%.
Fauna migrate as least as well as humans do, but flora are tied to their environment. The three determinants of floral population are mean temperature, available moisture, and distance from the equator.
Mean temperature tends to be a function of elevation – the higher a place, the colder, and the converse tends also to be true. Even the most rudimentary gardener knows that some plants can and others cannot withstand freezing temperatures.
Available moisture is self-explanatory. In the garden, some plants tolerate benign neglect and others do not. In nature, some grow close to streams and rivers which do not dry up, needing water, and others can thrive with water only in the fall and spring.
The unexpected factor here is the distance from the equator, which translates into how much available sunlight falls onto an area, and at what angle. We know from experience that the sun feels warmer on our skins when it is directly overhead than when it rises or sets. And earth’s axial tilt of 23.5 degrees means that more sun falls on the northern hemisphere in the time we northern-hemisphere dwellers call summer, less in the time we call winter.
The incident sunlight varies as well with longitude, that is, the distance from the equator: north to south. The equatorial areas have a supply of sunlight which varies very little around the year, but the farther you stray from the equator, the less incident sunlight is available. The supply of sunlight is nearly constant across bands of latitude, east to west.
What this means to human migration is that if travel takes place east to west, migration is very much less demanding than travel north to south. Every thirty miles of north-to-south travel, the local flora changes completely. In thirty miles of east-to-west travel, almost no changes will have taken place in the local flora. Those changes which have occurred will be gradual: a traveling band of humans will be able to predict what flora will be edible by its resemblance to flora they collected earlier in their journey. North-south travelers will have to conduct a series of careful experiments to arrive at knowledge of the local edible plants every thirty miles.
Throughout human evolution, thirty miles north or south was a generation’s migration. That distance could more than double across the easier east-west routes.
Human culture, and eventually civilization, must wait upon the establishment of survival. Thus we see that migration, with its huge advantages to east-west travel, shaped us almost from the beginning. To those who would travel north-south came difficulty after difficulty, perhaps shaping the nascent brain of the first problem-solvers more quickly than those who went east-west. This east-west group had the advantage of a semi-stable food source which allowed them to reproduce more quickly, spreading any evolutionary change faster through the gene pool.
Thus, in two separate ways, we are the product of our migrations.
Are we there yet?