Did Climate Change Make Us Human?
Given the ongoing controversy over global warming the question of whether humans can change Earth's climate is a familiar one. Lost in the fight over anthropogenic global warming is a more subtle and possibly more important question—how has climate changed people. In recent decades, the fossil record of hominin evolution and behavior, though still incomplete, has improved greatly. Triggered by a recent National Research Council (NRC) report, a perspective article in the journal Science poses the question, “did climate change shape human evolution?”
One fact is often overlooked when it comes to discussing humanity's place in nature, and that is that man is a part of nature. Indeed humans are a product of nature and the ever changing living system that controls the climate of planet Earth. In the brief for the NRC report “Understanding Climate’s Influence on Human Evolution,” the relationship that every species, including man, has with nature is spelled out:
All living things interact with the earth system—the combinations of land, atmosphere, and oceans—that make up the planet’s environment. As the earth system has changed over time, individual species have responded to these changes. In some cases, species have moved to new locations. In other cases, they have remained and adapted to the environmental changes, and sometimes this has led to the formation of new species. In some cases, species have become extinct.
Though many think that we humans somehow stand apart from nature in truth we do not. Homo sapiens, like any other species, has been shaped by the world around us. Scientists are just now beginning to understand how we evolved and the forces of nature that dictated that evolution. The basic premise is that large-scale shifts in climate alter the ecology and change resource availability. This leads to selection pressures which, in turn, may have influenced our evolution.
Skulls from some Hominins including H. sapiens.
Changes to the African climate are of great interest to anthropologists because that is where our species evolved. In “Climate and Human Evolution,” Peter B. deMenocal examines recent findings that may help verify the climate-human evolution link. According to deMenocal, climate change and its effects on African ecosystems may have played a key role in human evolution.
Notable hominin extinction, speciation, and behavioral events appear to be associated with changes in African climate in the past 5 million years. First appearance and extinction events, as well as key behavioral milestones, cluster between 2.9 and 2.6 Ma and again between 1.9 and 1.6 Ma. In the earlier group, these events include the extinction of Australopithecus afarensis (“Lucy”) near 2.9 Ma; the emergence of the robust australopiths (Paranthropus spp.), with large jaws and grinding teeth, near 2.7 Ma; and the emergence of the larger-brained Homo lineage sometime after 2.6 Ma, near the time when the first evidence for Oldowan stone tool manufacture, use, and transport appears.
You may have noticed that the term hominin has recently replaced the older, more familiar hominid in scientific articles. This is not a typo or some PC neologism but rather a reflection of anthropology's growing understanding of our human ancestors and other closely related species. According to archaeologist and science writer K. Kris Hirst, writing on About.com:
Up until the 1980s, paleoanthropologists generally followed the taxonomic system followed by the 18th century scientist Carl Linnaeus, when they spoke of the various species of humans. The family of Hominoids included the subfamily of Hominids (humans and their ancestors) and Anthropoids (chimps, gorillas, and orangutans). The problem is, recent molecular studies show that humans, chimps and gorillas are closer to one another than orangutans. So, scientists split the Hominoids into two subfamilies: Ponginae (orangutans) and Homininae (humans and their ancestors, and chimps and gorillas). But, we still need a way to discuss humans and their ancestors as a separate group, so researchers have proposed a further breakdown of the Homininae subfamily, to include Hominini (humans and their ancestors), Panini (chimps), and Gorillini (gorillas).
Basically, a hominin is what used to be call a hominid—a creature that is human or a human ancestor. This includes all of the Homo species (Homo sapiens, H. neanderthals, H. ergaster, etc.), all of the Australopithecines (Australopithicus africanus, A. boisei, etc.) and other ancient forms. By contrast, the term hominid includes all great apes, encompassing chimpanzees, gorillas, orangutans, and humans. Science, or at least scientific terminology, marches on. An updated Hominid/Hominin family tree is shown below:
A simplified Hominid/Hominin family tree..
That changing climate affects life is well accepted. For example, significant changes in flora and fauna occurred around 34 million years ago (Ma), when Earth cooled abruptly and extensive glaciers first developed on Antarctica. New lifeforms that appeared after 34 Ma were better adapted to the new environments that emerged: cooler polar regions, greater seasonality, and arid grasslands. As mentioned above, hominin development kicked into high gear during the run up to the Pleistocene Ice Age.
Once the Pleistocene Ice Age was in full swing, more important evolutionary developments occurred. Between 1.9 and 1.6 Ma the first hominin species to resemble modern humans—Homo erectus—appeared, with large brains, similar dentition, and the ability to craft refined stone tools. Also around 1.6 Ma, the first hominin exodus out of Africa and into Europe and South Asia began.
Of course, humans were not the only evolving lifeforms. The earlier appearance of modern C4 grasses led to a decline in forestation and the expansion of grasslands, particularly in Africa. This, in turn, led to the development of new animal species. According to deMenocal, “many new grazing bovid species appeared with specialized dentition (hypsodont molars) for processing the abrasive, grassy diet.” These developments and others are shown in the figure below.
African evolutionary and paleoclimate changes.
Not only were humans evolving, but the animals they would come to depend on were evolving as well. The evidence points to changing climatic conditions, and accompanying environmental changes, as driving factors in human evolution. The real question is, what was driving the changes in climate? Again quoting deMenocal:
African climate changes during the past 5 million years bear the signatures of two separate processes. Orbital precession forcing (with a period of ~20,000 years) acted as a “monsoonal pacemaker” that switched between wet and dry conditions. A long-term trend toward increasing drier and more variable conditions is superimposed on these wet-dry cycles, commencing after ~3 Ma and peaking near 1.8 to 1.6 Ma.
Orbital precession is one of the well known Milankovitch Cycles, which I have talked about many times before on this blog. What is being claimed here is that, on top of the cyclic variation from precession, there is a well established, long-term drying trend at work. This makes sense, since Earth's climate has cooled and a great deal of water has been locked up in glacial ice over the past 3 million years.
This trend should be kept in mind the next time a climate change alarmist starts babbling on about droughts being caused by global warming. It is true that climate changes can cause deserts to bloom and former forests and grasslands to become arid. Consider that, from 15,000 to 5000 years ago, the modern Saharan Desert was nearly completely vegetated, with large, permanent lakes and abundant wildlife. If desertification of the Sahara had happened recently humans would have most certainly gotten the blame. Indeed, there are those who do blame early Holocene climate change on humans (see “Humanity Blamed for 9,000 Years of Global Warming”).
The current theory is that precessional increases in summer radiation bolstered the monsoon, delivering more rainfall deeper into Africa. This enhanced the yearly Nile flood, sending greater amounts of river runoff into the eastern Mediterranean Sea, which led to organic-rich sediments (sapropels) being deposited on the seafloor. Stratigraphic samples going back many millions of years contain hundreds of these sapropel layers. Not coincidentally, these layers tend to appear in 100,000- and 412,000-year clusters that are linked to the modulation of orbital precession monsoon cycles and the eccentricity of Earth’s orbit, another of the Milankovitch Cycles. Dr deMenocal summarized the current thinking this way:
Hypotheses linking African climate and faunal change are constrained by these new observations. Faunal lineages typically persisted throughout dozens of wet-dry climate cycles, so it is unlikely that the orbital-scale variability alone was a selection agent. Similarly, early hypotheses emphasizing only the unidirectional development of open vegetation do not capture the now-evident complexity of African climate variability. An emerging view is that African fauna, including our forebears, may have been shaped by changes in climate variability itself. These views posit that increasing climate variability led to climate and ecological shifts that were progressively larger in amplitude.
All of this leads scientists to conclude that changes in Earth's climate, particularly since the onset of the Pleistocene Ice Age, created the world in which our ancestors evolved. Many anthropologists think the earliest humans arose on the open, arid plains of Africa. Before the climate changes discussed above, those open plains did not exist. But more than that, it was the climate's inconstancy that forced humans to evolve their unique set of skills—skills that distinguish us from other primates.
Man evolved on the open, arid plains of Africa.
We are creatures created by our environment, as are all the other lifeforms that share this planet. As recounted above, changes in climate are caused by long-term trends, primarily driven by geological changes in the planet's surface configuration, and medium term cycles dictated by Earth's orbital variation. In the short term there have been and will be aberrations—volcanic activity, asteroid impacts, gigantic releases of seafloor methane, etc.—but these have only transient effects on the Earth system. Man's mightiest efforts fall in the later category and can have only a fleeting impact on Earth's climate at best.
Earth's climate, and the changing of the climate itself, has helped to make us human. Today some are overly excited because they think we are changing that same climate. As stated in the NRC report: “In addition to responding to environmental factors, organisms also modify their environment, often in profound ways. Humans are no exception and have had a profound effect on the earth system.”
In other words, whatever effect we have on the environment it is all a part of nature, and nature is constantly adapting to change. Climate change alarmists and eco-wimps everywhere need to check their medication and chill out, because trying to stop climate change is like trying to stop evolution.
Be safe, enjoy the interglacial and stay skeptical.