Source: NOVA:"Becoming Human, Part 1: First Steps"
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This video segment adapted from NOVA: "Becoming Human" examines one method scientists use to understand ancient climate conditions. To test the idea that eastern Africa had undergone rapid swings in climate—from wet to dry to wet again—over a period that began 10 million years ago, German scientists studied the fossils of tiny one-celled aquatic organisms called diatoms. Knowing that white layers of a rock formation consist of deep-water diatoms, and darker layers consist of shallow-water diatoms, the scientists have interpreted the alternating layers in the formation to mean that a massive lake appeared and disappeared many times in their study area. If this part of Africa indeed experienced wet and dry periods over time, this supports a new idea that suggests climatic variability may have shaped human evolution.
Download and ShareFor more than a century, scientists had a simple idea about the connection between climate and human evolution. But evidence in the form of microscopic organisms called diatoms might be about to change that idea.
It's a long-held belief by many that a steadily drying eastern African climate forced our ancestors out of the jungles they used to inhabit. By coming down from the trees and onto an expanding grassland, or savanna, humans became bipeds, walking on two legs instead of four. Brain size subsequently increased, and humans developed survival skills, such as using their two free hands to make tools. In this way, according to proponents of this theory, our species of "modern" humans—Homo sapiens—split from other species of early humans.
But a more recent hypothesis suggests that eastern Africa's past climate was highly variable—from wet to dry to wet again—with swings occurring as little as 1,000 years apart. According to this hypothesis, it's this climatic variability that may have shaped human evolution. Among the evidence used to support this big new idea are tiny single-celled organisms called diatoms. Diatoms are microscopic algae with shell-like cell walls called frustules, which are made of silica. Diatoms record evidence of past environmental conditions in these shells. As the frustules form, they take on characteristics of the water around them. Shells grown in shallower waters contain more of a heavier form of oxygen isotope. And as the German scientist featured in the video discovered, diatoms that live in deeper water tend to have much lighter, stable isotope composition in their shells compared with those that live in shallower water.
Scientists apply what they know of diatoms living today to diatoms that lived in the past. From this information, they can infer ancient climate conditions. Some species of diatoms prefer deep-water conditions, while others prefer shallow-water or nutrient-rich conditions. By examining which species of fossil diatoms are contained in a layer of sediment, scientists can infer the water conditions in which these organisms lived. Changes in species composition over time can tell scientists about the changes in the water depth and quality. This is one important line of evidence in developing a climate reconstruction for a region. It means that it is likely that the climate was drier when the lake was shallower, and then wetter when the lake was deeper.
Studying the period during which humans were evolving, the German scientists identified at least three times since drying began 10 million years ago when the lake in eastern Africa was deep enough to fill the valley around it. This means that in this part of Africa, coinciding with a critical period of human evolution, climate shifted again and again: droughts gave way to monsoons, and monsoons gave way to droughts. In this way, microscopic diatoms have helped support a compelling idea that rapid shifts between periods of wet and dry might have provided stress that caused a new subspecies of human—ours—to evolve.
Here are suggested ways to engage students with this video and with activities related to this topic.
For more media and information about the topics in these teaching tips, see these links:
To learn more about how the Grand Canyon's exposed rock layers reveal its climate history, check out The Grand Canyon: Evidence of Earth's Past.
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