Climate change paved way for emergence of humans

Man is the only bipedal mammal. Though we prefer to see ourselves as elevated, cultured and refined, we are primates, members of the ape family that includes gorillas and chimps.

Although much advanced, we undeniably share many anatomical and behavioral attributes with our distant primate cousins. Human evolution is well represented in the fossil evidence. Geochronologists can date physical remains with some precision, and physical anthropologists have established distinct anatomical similarities along parallel timelines.

There are few clear “straight lines” of descent. Branching, parallel developments and dead ends leave uncertainty about precise details of the human lineage.

Different theories of human evolution have different details, but all lead along the same path.

There is little doubt that changing climate was an important factor in human evolution, since generally adaptations tend to equilibrate with environmental changes over time.

The Great African Rift, a continent-size tectonic gash, began between 5 million and 10 million years ago. Magma from Earth’s mantle pushed upward on the crust, uplifting the mountains of East-Central Africa. The rift grew wider as it tore Africa apart. As it widened, volcanic activity eased the pressure, causing it to sink along a series of steplike faults that bordered deep valleys.

Around 7 million years ago chimpanzee and hominid lineages branched. In the northern rift valleys the land began to rise even faster, forming what is now the Wall of Africa. These mountain ranges caused precipitation that robbed moisture from the air, causing increasing desertification downwind.

The resulting climate change favored the evolutionary emergence of humanity.

As the rain forests slowly gave way to savanna, apes that could move longer distances between trees, and that could stand up to see above the tall grasses, had an adaptive advantage.

For 3 million years the fossil record is sparse, but by 4 million years ago shoulders had broadened and moved rearward. Hands shortened and developed the prehensile thumb, which led to the use of tools (the power grip) and throwing (the fine grip) by 2 million years ago.

The power grip could hold tools to dig tubers and other roots, while the fine grip could hold rocks and spears to increase throwing distances, which allowed better hunting efficiency.

Nutritious tubers and concentrated protein from meat contributed to the rapid increase in size of the human brain, which peaked about 100,000 years ago during the 5 million-year transition from quadruped ape to biped homo sapiens.

Two other anatomical innovations combined, allowing us to throw overhand.

The human shoulder allows the arm to move in any direction through a wide arc, while tendons act as springs to store enough energy to accelerate a 5-ounce baseball.

Our long, flexible waist allows the torso to rotate more, storing even more energy to impart to a rock, a spear or a baseball at 100 mph.

It is one of our distinguishing physical characteristics. Apes cannot throw fastballs.

———
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.