New study captures 6M years of African mammal fossil history
Buffalo graze during the dry season. Photo by Kaye Reed
The East African Rift Valley is a fossil-rich area, reaching across Ethiopia, Kenya and Tanzania, that preserves the most complete record of human evolution anywhere in the world — including the 3.2-million-year-old fossil skeleton “Lucy,” discovered 50 years ago in Hadar, Ethiopia.
Over the past 6 million years, the valley has been home to vast numbers of mammals, whose fossils have also been collected and catalogued, and who have many modern analogues living on the African plains. In fact, mammals comprise the bulk of fossils discovered in the valley.
However, we have little knowledge of how long-term changes or trends in the diversity, size and migration of these animals may have influenced hominin — early bipedal ancestors — diversity and distribution.
A new study published in Nature Ecology & Evolution examined the presence and absence of species over 6 million years to reveal that, at various times, endemic animals — those that were found in only one region — disappeared from the fossil record, a long trend that likely resulted from some mammals being able to travel to new regions as immigrants rather than evolving there.
This research began in the Arizona State University Institute of Human Origins Paleoecology Lab led by Kaye Reed, a research scientist at the Institute of Human Origins (IHO) and an emeritus professor of the School of Human Evolution and Social Change at ASU. A cohort of ASU graduate students, now doctoral graduates — including John Rowan, Ellis Locke, Irene Smail and Ignacio Lazagabaster, as well as Jason Kamilar, previously at Midwestern University, and IHO research scientist Chris Campisano — collaborated across several years to build a database of fossil and modern mammal bone collections that was used in this analysis.
“For the last three decades, there has been intense debate about patterns of species diversity and geographic distributions in eastern Africa’s hominin fossil record. Our team’s analyses of the mammal fossil records from this region help to address these issues,” said Rowan, assistant professor of human evolution at Cambridge University.
“Although hominins are a rare component of the fossil record, they were just another ‘critter’ on the landscape for much of their evolutionary history," said Campisano, an associate professor in the School of Human Evolution and Social Change. "Analyzing large, regional datasets of other fossil fauna such as this is a tool that can help us make predictions regarding the patterns of hominin diversity and distribution over time within the context of environmental change.”
Over the course of 6 million years, environmental changes occurred, shifting the valley from woody habitats that supported a wide variety of mammals — including carnivores, omnivores like pigs and suids, browsers and frugivores including monkeys and other primates, hooved grazers and larger mixed feeders like elephants — to a drier, more open landscape, and more grazing animals figured prominently.
Groups of migrating mammals may have outcompeted the established species in resource acquisition, which then led to their extinction. These new groups are found in different regions of eastern Africa but became more similar through this time span, although at different times and through different climates and habitats.
“Our ancestors were surely influenced by the same environmental factors as other mammals,” Rowan said. “When the early hominin fossil record is thin or puzzling — and this is the case more often than not — it is useful to draw on the fossil records of other mammal groups for comparison and for contextual clues.”
We can see that this also happened in the human lineage as hominin species became more widespread but ended in the more recent past with only Homo sapiens across Africa.
"This research dives into how climate change, paleoenvironments and mammal evolution have intertwined in eastern Africa over the last 6 million years,” said Lazagabaster, Ramon y Cajal Researcher at the Department of Paleobiology, National Research Center on Human Evolution, CENIEH, Spain. “By exploring these connections, it lays out a key biogeographic framework for studying hominin evolution, providing critical contextual insights to test hypotheses about the ecological adaptation, evolution and diversity of our early hominin ancestors."
Did climate change cause these more prevalent animal species to have the ability to move further and basically colonize regions through corridors of similar habitat? Perhaps, and it is something that can be further examined with new research based on what the authors have discovered.
“This research is a great example of how we can leverage a wealth of geographic and temporal data to generate novel insights into the evolution of humans and the mammals that lived alongside us in Africa,” said Kamilar, now a professor at the University of Massachusetts Amherst.
'Lucy' discovery turns 50: IHO celebrates 'A Year for Human Origins'
This year, ASU's Institute of Human Origins is celebrating the 50th anniversary of the discovery of the fossil skeleton “Lucy” — one of the most significant anthropological findings of our time.
By reflecting on the importance of that discovery, the institute aims to reignite a global interest in how we “became human,” promoting the importance of connecting our human past to the planet's global future. “A Year for Human Origins” will bring awareness of our species’ success — through innovation, cooperation and a shared tenacity for survival — as residents of Earth who started in Africa millions of years ago.
IHO Founding Director Donald Johanson will be in conversation with New York Times columnist Carl Zimmer at the 92nd Street Y in New York City on Nov. 14, talking about the impact of Lucy on human-origins science and the importance of connecting the human past with the global future.
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