Obstacles to North-South Spread in the Americas and Africa

Key Insight

Diffusion along north-south axes in Africa and the Americas faced significant hurdles, primarily due to drastic changes in latitude-dependent climate. For instance, Fertile Crescent crops, after reaching Egypt and Ethiopia, were halted by 2000 miles of tropical conditions, preventing their spread to South Africa's otherwise suitable Mediterranean climate. Similarly, tropical African crops could not cross South Africa's Fish River due to unadapted Mediterranean conditions. In the Americas, hot intervening lowlands of Central America prevented Andean domesticates like llamas, guinea pigs, and potatoes from spreading northward to Mexico's highlands, and Mexican turkeys and sunflowers from spreading southward to the Andes.

The north-south spread was notably slow: less than 0.5 miles per year for corn from Mexico to the U.S. Southwest, and 0.2 miles per year for llamas from Peru to Ecuador. This resulted in incomplete suites of domesticates; Mesoamerica never acquired Andean mammals, and different, often related, species of crops like cotton, chili peppers, and squashes were independently domesticated in Mesoamerica and South America. This pattern, evidenced by multiple independent domestications for many American crops (e.g., lima beans, common beans, chili peppers domesticated on at least two separate occasions), contrasts sharply with the predominantly single domestications observed for Southwest Asian crops, indicating a much slower spread that failed to preempt local domestication elsewhere.

Beyond latitude, topographic and ecological barriers further impeded north-south diffusion. Deserts in Texas or between Indonesia and Australia, or the narrowness and lack of high-elevation plateaus in Mesoamerica, restricted crop exchange. In Africa, trypanosome diseases carried by tsetse flies stopped the southward advance of animals like horses and significantly slowed cattle, sheep, and goats for 2000 years, with these livestock only reaching South Africa between A.D. 1 and 200. These combined barriers meant that, unlike Eurasia's rapid agricultural spread, Native American and sub-Saharan African agricultural systems often remained geographically confined, influencing the subsequent slower diffusion of other technologies and contributing to divergent historical trajectories.