Human ancestors evolved in ecosystems unlike any found today

To understand the environmental pressures that shaped human evolution, researchers must first piece together the details of the ancient plant and animal communities that our fossil ancestors lived in over the past 7 million years.

Putting together the puzzle of millions-of-years-old ecosystems is a difficult task, and studies therefore tend to reconstruct the environment by drawing analogies with present-day African ecosystems. A new study published in the journal Proceedings of the National Academy of Sciences calls into question such approaches and suggests that the vast majority of human evolution occurred in ecosystems unlike any found today.

To test for differences between modern and ancient environments, the researchers analyzed a dataset of more than 200 present-day African mammal communities and more than 100 fossil communities spanning the past 7 million years in eastern Africa.

They found that prior to 700,000 years ago, mammal communities looked far different from those today. For example, fossil communities supported a greater diversity of megaherbivores, species over 2,000 pounds, such as elephants. Likewise, the dietary structure of fossil communities frequently departed from those seen today, with patterns of grass- and leaf-eating species fluctuating in abundance.

Around 1 million years ago, fossil communities began transitioning to a more modern makeup, which the authors suggest is the likely the outcome of long-term grassland expansion coupled with arid climate pulses. The new paper adds to growing evidence that scientists need to critically re-evaluate our understanding of the ancient ecosystems in which early humans evolved.

“For a long time, our field has been trying to pin down how environmental changes influenced human evolution, but we’ve got to be able to reconstruct past environments right in the first place,” said lead author Tyler Faith, curator of archaeology at the Natural History Museum of Utah. “If we continue to reconstruct ancient environments on the basis of modern African ecosystems, we are likely missing an entire realm of possibilities in how past ecosystems functioned. Our study invites our fellow researchers to think more critically about that.”

For example, modern African ecosystems are dominated by ruminants–relatives of cows and antelopes that have four compartments in their stomachs to thoroughly break down food. Non-ruminants equipped with simple stomachs are comparatively rare, with at most eight species coexisting in the same area today. Non-ruminants, including relatives of elephants, zebras, hippos, rhinos and pigs, are like digestive conveyor belts. They eat larger quantities of plants to make up for their inefficient digestion. In contrast to the present-day pattern, eastern African fossil records document landscapes rich in non-ruminant communities, with dozens of species co-existing within the same area.

Fossil and modern communities were also vastly different in terms of body sizes. The fossil records document lots more megaherbivores than their modern counterparts. A steady decline of megaherbivores began 4.5 million years ago until they represented a more modern distribution 700,000 years ago.

“These ancient herbivore communities were probably consuming far more vegetation, which means less fuel for wildfires. Because fire is an important part of modern ecosystems in Africa and favors grasslands over woodlands, it’s going to fundamentally alter how things are working at the level of entire ecosystems, starting with the plant communities,” adds John Rowan, co-author and postdoctoral researcher at the University of Massachusetts Amherst. “Paleontologists have been aware of that, but until now, no one’s really tried to measure just how different the past was compared to the present.”

The authors do not fault previous researchers for relying so heavily on analogies with present-day African ecosystems, emphasizing that a study of this scope has only recently become possible.

“Paleontology has hit a big data era,” said Faith. Co-author and Colorado State University assistant professor Andrew Du added, “With the assembly of large, comprehensive datasets, we can now ask important questions that are fundamentally different from those asked in the past. We can investigate larger-scale patterns and dynamics that undoubtedly influenced the course of human evolution.”

Research article: Early hominins evolved within non-analog ecosystems

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