Human Ancestors Not to Blame for Ancient Mammal Extinctions in Africa

New research disputes a long-held view that our earliest tool-bearing ancestors contributed to the demise of large mammals in Africa over the last several million years. Instead, the researchers argue that long-term environmental change drove the extinctions, mainly in the form of grassland expansion likely caused by falling atmospheric carbon dioxide (CO2) levels.

Tyler Faith, curator of archaeology at the Natural History Museum of Utah and assistant professor in the Department of Anthropology at the University of Utah, led the study. The research team also included John Rowan from the University of Massachusetts Amherst, Andrew Du from the University of Chicago, and Paul Koch from the University of California, Santa Cruz.

The study was published in the journal Science. Faith said:

Tyler Faith surveying Pleistocene outcrops in western Kenya, where he has conducted fieldwork since 2009. (Image: J. Tyler Faith)

Tyler Faith surveying Pleistocene outcrops in western Kenya, where he has conducted fieldwork since 2009. (Image: J. Tyler Faith)

To test for ancient hominin impacts, the researchers compiled a 7-million-year record of herbivore extinctions in eastern Africa, focusing on the very largest species, the so-called “megaherbivores” (species over 2,000 lbs). Though only five megaherbivores exist in Africa today, there was a much greater diversity in the past.

The decline of African megaherbivore diversity (gray curve) over the last seven million years was driven by falling atmospheric carbon dioxide and the expansion of grasslands, not ancient hominin impacts. The onset of the megaherbivore decline around 4.6 million years ago (red dashed line and shading) occurs well before the appearance of tool-bearing hominin species capable of hunting large prey. (Image: John Rowan) Read more at: https://phys.org/news/2018-11-human-ancestors-blame-ancient-mammal.html#jCp

The decline of African megaherbivore diversity (gray curve) over the last 7 million years was driven by falling atmospheric carbon dioxide and the expansion of grasslands, not ancient hominin impacts. The onset of the megaherbivore decline around 4.6 million years ago (red dashed line and shading) occurs well before the appearance of tool-bearing hominin species capable of hunting large prey. (Image: John Rowan)
Read more at: https://phys.org/news/2018-11-human-ancestors-blame-ancient-mammal.html#jCp

For example, 3-million-year-old “Lucy” (Australopithecus afarensis) shared her woodland landscape with three giraffes, two rhinos, a hippo, and four elephant-like species at Hadar, Ethiopia. When and why these species disappeared has long been a mystery for archaeologists and paleontologists, despite the evolution of tool-using and meat-eating hominins getting most of the blame. Faith said:

Taking a closer look

Faith and his team quantified long-term changes in eastern African megaherbivores using a dataset of more than 100 fossil assemblages spanning the last 7 million years. The team also examined independent records of climatic and environmental trends and their effects, specifically global atmospheric CO2, stable carbon isotope records of vegetation structure, and stable carbon isotopes of eastern African fossil herbivore teeth, among others.

Researchers looked at more than 100 sites with rich fossil records to track the long term decline of megaherbivore diversity in East Africa. (Image: J. Tyler Faith)

Researchers looked at more than 100 sites with rich fossil records to track the long-term decline of megaherbivore diversity in East Africa. (Image: J. Tyler Faith)

Their analysis reveals that over the last 7 million years, substantial megaherbivore extinctions occurred — 28 lineages became extinct, leading to the present-day communities lacking in large animals. These results highlight the great diversity of ancient megaherbivore communities, with many having far more megaherbivore species than exist today across Africa as a whole.

A fossil tooth of a hippo (Hippopotamus amphibius) (left) and a fossil tooth of a white rhinoceros (Ceratotherium simum) (right) , two of the few surviving megaherbivores, from the Late Pleistocene of western Kenya (left). (Image: J. Tyler Faith)

A fossil tooth of a hippo (Hippopotamus amphibius) (left) and a fossil tooth of a white rhinoceros (Ceratotherium simum) (right), two of the few surviving megaherbivores, from the Late Pleistocene of western Kenya (left). (Image: J. Tyler Faith)

Further analysis showed that the onset of the megaherbivore decline began roughly 4.6 million years ago and that the rate of diversity decline did not change following the appearance of Homo erectus, a human ancestor often blamed for the extinctions. Rather, Faith’s team argues that climate is the more likely culprit. John Rowan, a postdoctoral scientist from University of Massachusetts Amherst, said:

The loss of massive herbivores may also account for other extinctions that have also been attributed to ancient hominins. Some scientists suggest that competition with increasingly carnivorous species of Homo led to the demise of numerous carnivores over the last few million years. Faith and his team suggest an alternative. Paul Koch said:

Faith concluded, saying:

Provided by: University of Utah [Note: Materials may be edited for content and length.]

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