LA JOLLA, CA. – Anyone who’s taken an earth science class knows that environmental shifts, some of them quite rapid, have driven the evolution, and the extinction, of species. Scientists now are bringing into greater focus just how much climate change has shaped human evolution.
The influence of climate shifts on early human history was the focus of a recent afternoon symposium at the Salk Institute for Biological Studies in La Jolla, Calif.
Overlooking the expansive Pacific as a spring rainstorm cleared through the area, Salk was an inspiring venue for reviewing the deep history of humankind and for considering how climate change has played, and continues to play, a role in human destiny.
The gathering was hosted by the Center for Academic Research and Training in Anthropogeny (CARTA), which studies human origins. This multi-disciplinary group, based in San Diego, has hosted symposia on a wide range of topics: how language evolved; male aggression and violence in human evolution; the evolution of human nutrition; the evolution of human altruism; the origins of the human brain; and understanding human origins and the implications for modern medicine – among many other subjects.
The spring “Human-Climate Interactions and Evolution: Past and Future” symposium offered a mix of scholarship and personal perspectives from an all-star line-up of researchers.
The symposium began with presentations from researchers who study human origins, and it progressed to talks by scholars who study today’s climate changes.
Peter B. deMenocal, a professor in the Department of Earth and Environmental Sciences at Columbia University, described how major shifts in African climate had coincided with two key developments in human evolution.
The first, from between 2.9 million years ago to 2.4 million years ago, marked big changes in the family tree. That was the period during which the ancestral lineage of Australopithecus Afarensis, commonly referred to as “Lucy”, became extinct. Two other groups appeared: the first members of our own genus, Homo, and another now extinct group, Paranthropus.
The second development occurred between 1.9 million and 1.6 million years ago. Homo erectus, with its taller and thinner frame, became the first to leave Africa and populate Southeast Asia and Europe. This period also saw big advances in stone tools.
Rapid shifts between wet and dry periods
These two periods were marked, after long periods of climate stability, by episodes of rapid shifts between wet and dry periods, transforming the landscape of early humanity into increasingly dry and open grasslands – and leading to the African savanna familiar today. This transformation required early Homo to adapt quickly to changes in the environment.
The “pacemaker of African paleoclimate”, deMenocal said, was regular pendulum swings of wet and dry cycles occurring about every 20,000 years as Earth “wobbled” about its axis of rotation – as it does today. Because of this “precession,” the star Vega about 10,000 years ago, was our “north star.”
Eccentricities in the shape of Earth’s orbit around the Sun and changes in the tilt of its axis of rotation, along with precession, have been the major drivers of climate change over geologic time, and all these have had impacts on human evolution.
Periods of high variability in climate, driven by these large-scale orbital cycles, have fueled dramatic evolutionary changes throughout history.
“It turns out that almost everything that’s interesting in African human evolution is concentrated in those periods of high climate variability,” said paleoanthropologist Rick Potts, who directs the Smithsonian Institution’s Human Origins Program in Washington D.C.
“Even the origin of every single major technological and behavioral transition in human evolutionary history is focused in one of these high climate variability intervals,” he said. The history of human evolution and survival is punctuated by periods of adaptation to environmental change “unlike our other bipedal ancestors, who didn’t survive.”
In another interesting talk, climate scientist William Ruddiman, professor emeritus at the University of Virginia, discussed when humans are thought to have begun having a detectable impact on the global environment, ushering in what’s been dubbed the “Anthropocene.”
Thousands of years of human impacts
Some say humans began leaving an enduring impact with the atomic bomb tests in the 1940s and ’50s. But Ruddiman and others maintain that humans really began to have an enduring global impact thousands of years ago through the widespread dispersal of livestock and crops.
In fact, Ruddiman said, early farming and agricultural burning around 2,000 years ago halted what otherwise would have been a downward slope of average global temperatures toward the next ice age.
He presented graphs showing atmospheric methane and CO2 peaking about 10,000 years ago, and then declining. But about 6,000 years ago, CO2 began to come back, followed by methane about 5,000 years ago.
The recorded rises in greenhouse gases coincide with the historical expansion of agriculture in the Middle East, Europe, the Americas, and China – and importantly in Southern China and the Philippines, where people developed irrigated rice patties. Essentially man-made swamplands, rice patties are a major source of atmospheric methane, Ruddiman pointed out.
“In the last three to four years there’s been an explosion of evidence, ground truth evidence, that people (were) doing things that (were) putting a lot of methane and carbon dioxide in the atmosphere,” Ruddiman continued.
“We have a long history of affecting climate, and the scope of it is still coming into focus, but it’s coming into focus pretty fast. I think humans have been a major factor on the landscape for thousands of years, not just 200 years.”
Long recovery times for biodiversity
Stanford biologist Elizabeth Hadly, offered a grim view of how current climate change is expected to affect species around the globe.
Global biodiversity did not recover from Earth’s past mass extinction episodes for five to 10 million years – and the biodiversity that has followed has been profoundly different from what came before, Hadly said.
With a projected 2 degree C (3.6 degrees F) rise in average global temperatures above preindustrial levels, “we will have global planetary temperatures warmer than our species has ever experienced on our time on Earth,” Hadly continued. “And if we happen to make it to 2100 and to temperatures somewhat above 4 degrees (7.2 degrees F), Earth has not been as hot as that projected temperature in 14 million years. … Mammal species don’t live that long. We don’t have mammal species on the planet that are 14 million years old. So the memory of this climate in the genomes of these species is gone.”
Scant prospects for ‘politics as usual’
Naomi Oreskes, professor of the History of Science and an affiliated professor of Earth and Environmental Sciences at Harvard, emphasized in her talk that the climate challenge must be viewed as a political challenge.
“Without talking about political systems and governance, I don’t know how we can solve the problems we face,” she said.
“What may actually determine the question of how we survive [climate change] – is whether we can create and sustain a disruptive politics,” Oreskes continued.
“Politics makes nearly all scientists very uncomfortable. We don’t want what we do to be politicized, and we are afraid of being seen as ‘political’. [ycc-tweetable-text tweetable=”false”]But it seems very unlikely that we can solve climate change with politics as usual.[/ycc-tweetable-text]”
Moral issue: Leave 3-billion under-served behind?
In the final presentation of the day, Scripps Institution of Oceanography climate scientist Veerabhadran Ramanathan, who has met with world religious leaders in attempts to engage faith groups on the climate issue, argued that the climate challenge will most adversely affect the world’s three billion most under-served people.
“We have two separate but co-dependent worlds,” Ramanathan said. One-billion people live with seemingly unlimited fossil fuels, and they are responsible for 50-70 percent of the greenhouse gas emissions, he said. The most needy three billion, meanwhile, lack access to fossil fuels even for cooking.
In talks at the Vatican, Ramanathan said, those who had attended concluded that the way forward requires fundamentally changing our attitude toward each other and toward nature.
“By leaving three billion behind, [those 3 billion people] will suffer the consequences of our fossil fuel consumption… so it’s a moral issue,” he said.
Ramanathan concluded his talk with a prediction that by 2050, global average temperatures will have climbed by 2 degrees C (3.6 degrees F). Climatic conditions will have will become so oppressive – with extreme storms, droughts, floods and more – that people will change attitudes. “We will decarbonize the (global) economy,” he said.
‘A grand societal project … a world of our own making’
Wrapping up the meeting was Potts, who conveyed a sense of awe at how far the human species has come, with an incredible capacity to adapt to changing surroundings and challenges – but also with a sense of worry and dread.
“[ycc-tweetable-text tweetable=”false”]Now we find ourselves where the planetary scale of human influence is unquestionable[/ycc-tweetable-text],” Potts said. “We have the rapidity of change in landscape that is quite different from anything that has ever been experienced before in human history. The planet is packed with people. Our ways of changing our immediate surroundings and also … the atmosphere and oceans affect people we have never met and will never meet.
“We have before us a grand societal project, and that grand societal project is to be carried out by the first species that has awareness of extinction – including its own extinction but also the extinction of other forms of life on Earth.
“It seems to me that many of the problems we have addressed here haven’t yet brought up the matter of whether the scale of the problem can be matched by the scale of our compassion. We are a species capable of developing principles, of living in a purposeful and meaningful world, and acting according to those principles and values – depending upon how much we care.”
“The future will be a world of our own making.”