Human Evolution & Climate Change

Submitted by Gary Snider, President, Eastman Humanist Community

One of my favourite writers on the topic of human evolution is William H. Calvin, a neurophysiologist from the University Of Washington School Of Medicine in Seattle. At age 79 he still teaches, travels extensively and writes on the great questions: Where did we come from? What are we? Where are we going? In his 2002 book “A Brain For All Seasons – Human Evolution and Abrupt Climate Change” he tackles the first two questions and offers interesting insights into our likely futures.

One of the appealing aspects of Calvin’s writing is its accessibility. He brings the reader along on his journey to Africa, (via Darwin’s home near London and a family reunion in Germany) to see places where great breakthroughs in our understanding of human origins occurred. This is more than a science history travelogue, and Calvin is both good company and a great tour guide, pulling in insights from brain science, anthropology, atmospheric climate and oceanography. At every stop along the way the weft and weave of what makes us human is illustrated with interesting stories and a good deal of wit.

For those who might not get around to reading this book, but are nonetheless interested in human evolution, I’d like to share a few bits that I found intriguing:

  1. Perhaps the biggest question, I had coming to this book was: “Why humans?” Why didn’t other omnivores like raccoons or bears evolve into big brained, socially complex beings too? And what about all the other primates (orangutans, chimps and bonobos) – what held them back? Calvin wastes no time laying out the canvas on which our human journey began. About 5 million years ago, the hominid line moves away from chimps and bonobos and the extant bone and footprint evidence shows that Australopiths, the first hominids, by 4 million years ago got around primarily by walking upright. Calvin postulates that this ability and a change in diet, from mostly veggies to one that increasingly included meat, propelled us along a different path. So when hard times came, in the form of a changing climate (think brush fires, drought and poor foraging for berries and fruit), our ancestors learned how to successfully scavenge the kills of big predators and eventually to be predators ourselves. While our primate cousins retreated to smaller habitats, our ancestors were able to eat grass, second-hand, by eating grazers.
  2. In Africa, our human ancestors repeatedly faced bottle-neck crises when climate moved rapidly from warm/wet to cool/dry periods. These cool/dry periods were drought times and forced our ancestors to live closer to rivers and waterholes. Being bipedal had great advantages where wading and tree climbing got us closer to food and away from predators. These bottleneck events also helped to cement the most useful adaptations and at some point, they led our ancestors to something that actually modified how our brains worked…
  3. Throwing. That’s right, the location in our brain that has the biggest role in helping us coordinate our arm and hand movements is also the part of the brain that deals with language. As we hung out in our watery refuges, we learned that a branch or rock tossed in amongst a herd of grazers, tightly packed together as they came for water, would occasionally provide a big payout. With lots of time, accuracy improved, and our brains improved too, doubling in size in just 1.5 million years.  Language could very well have been a hitchhiker that came along with precision in hand and arm movement. And language, even very simple proto language, is useful for organizing a lot of activities. So, what came along as a side benefit of throwing turned out to be a lever that further pushed our evolution.
  4. The Hand Axe: What was homo erectus doing with these odd things? They are roughly tear drop shaped, relatively flat, and range in size from dessert to salad plates and are worked to a sharp edge all around. In many places in the Rift Valley of Kenya they show up by the hundreds, concentrated locally in sediments of 1 to 1.8 million years. Were they an axe, a scraping tool, a core for making smaller blades, an anvil, a killer Frisbee? Calvin thinks the latter is most likely, but they were thrown edge up, to land vertically into the grazer herds at waterholes.
  5. The glacial period that took hold 250,000 years ago ushered in a “chattering age” with sudden swings in climates. The grassy Savannah that was the Sahara dried up and this pushed our ancestors into Asia and beyond. While the dry times were disastrous for some species, they were often boom times for humans who now followed herds of grazers. Dry times replaced forests with grasslands and as herd populations boomed, so did our ancestors.
  6. One of the challenges in hunter-gatherer groups is freeloading. It’s widely held that successful hominid hunters shared meat with both kin and non-kin, similar to the meat-sharing we see in chimps today. This altruistic behavior creates stronger social bonds but also leaves the group vulnerable to freeloaders. Language complexity came to the rescue. One theory is that the categories needed for argument in syntax arose in order to track benefits given and payback received. “But you owe me!” requires the categories of actors, recipients and benefits – complex language!

The book wraps up with Calvin warning us about where we’re likely headed.

As temperatures rise, large inflows of freshwater from the melting ice sheets atop Greenland and Antarctica have diminished and will eventually stop the sinking saline gyres in the Atlantic. These ocean currents appear to play an important regulatory role in the planet’s climate, as their presence supports the warm/wet periods (like the one that we’re enjoying now) and their absence extends the cool/dry periods (like the one that had a mile-high ice sheet on top of Manitoba). With over 7 billion people on our planet, the next bottle neck event has the very real chance of being a closing chapter for humans and our amazing evolution.

But the knowledge and creativity we’ve gained via past climate disruptions could serve and save us, even now. Calvin is ultimately hopeful that we can survive and thrive, if only we use our future predicting abilities and capacity for problem solving and planning wisely. That’s up to you and me!