Monday, June 17, 2024

Large mammals shaped the evolution of humans in Africa

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That humans originated in Africa is widely accepted. But it’s not generally recognised how unique features of Africa’s ecology were responsible for the crucial evolutionary transitions from forest-inhabiting fruit-eater to savanna-dwelling hunter.

These were founded on earth movements and aided physically by Africa’s seasonal aridity, bedrock-derived soils and absence of barriers to movements between north and south.

These features promoted extensive savanna grasslands marked by erratic rainfall, regular fires and abundant numbers of diverse grazing and browsing animals. My lifelong studies have focused on the ecology of Africa’s large herbivores and their effects on savanna vegetation.

In his recent book, Norman Owen-Smith, University of the Witwatersrand, by linking pre-existing threads together for the first time, explains how distinctive features of these animals’ ecology, founded on Africa’s physical geography, enabled the adaptive changes that led ultimately to modern humans. What emerges is the realisation that this amazing evolutionary transformation could only have occurred in Africa. This recognition emphasises the deep cultural legacy formed by Africa’s large mammal heritage for all of humankind.

Starting during the late Miocene, around 10 million years ago, a plume of molten magma, hot liquid material from deep inside the Earth, pushed eastern parts of Africa upward. This led to rifting of the Earth’s crust, volcanic eruptions and soils enriched in mineral nutrients from the lava and ash. Grassy savannas spread and animals adapted increasingly to graze this vegetation component. Apes from that time were forced to spend less time up in trees and more time walking upright on two legs.

Homo habilis
Around 2.8 million years ago, another lineage split off from the australopithecines, reversing the trend towards robust dentition.
This lineage used stones chipped to serve as tools. These were used to scrape flesh from carcasses of animals killed by carnivores, and crack open long bones for their marrow content. This transition in ecology was sufficiently momentous to warrant a new generic name: Homo, specifically habilis (“handy-man”).

Homo erectus
Several hundred thousand years of progressive advancements in upright walking and brain capacity led to the next major adaptive shift, exemplified by improvements in the design of stone tools. Stone cores became shaped on both sides to aid the processing of animal carcasses. This led to the emergence of Homo erectus around 1.8 million years ago. These early humans had become efficient hunters. Consequently, meat and bones became reliable food resources year-round.

Homo sapiens
After 8,00,000 years ago, fluctuations in heat and aridity became more extreme in Africa. Finely crafted stone tools defined the transition into the Middle Stone Age, coupled with the emergence of modern Homo sapiens in Africa around 300 thousand years ago. But despite its hunting prowess Homo sapiens had declined to precarious numbers in Africa by around 1,30,000 years ago, following an especially severe ice age. Genetic evidence indicates that the entire human population across the continent shrank to fewer than 40,000 individuals, spread thinly from Morocco in the north to the Cape in the far south. One remnant survived by inhabiting caves along the southern Cape coast, exploiting marine resources.

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