My Energy Journey – Part 7
Becoming human
Earth’s life is very fortunate that its star is the Sun. The Sun is a big star. There are much larger stars, but the Sun is larger than 95% of the stars in the visible universe. The bigger the star, the faster it burns out, so the Sun is in a sweet spot of star sizes, which enabled it to burn very stably over billions of years while producing a lot of energy to support Earth’s life. Complex life would certainly not exist, if not for that stability. Much larger stars would have already met their end, and smaller stars, the red dwarfs, might be marginal hosts for life. White dwarfs also have problems, although there is recent excitement that they may be better hosts for life as we know it than previously thought.
As it is, complex life is relatively fragile, which is why so many mass extinctions have punctuated the eon of complex life. Ecosystems with animals in them have what are called food chains, with photosynthesizers at the base, various levels of grazers, symbionts, parasites, and detrivores (which eat dead organisms) above that, and predator levels at the top. Each level loses about 90% of the energy of the previous level in becoming the consuming organism. If one level of the food chain is somehow impaired, the entire food chain above that level can collapse.
While there has been a lot of scientific enthusiasm to credit celestial events with mass extinctions in recent generations, only the bolide that wiped out dinosaurs is widely accepted as causing a mass extinction. The Sun has been slowly brightening over the eons, and carbon-dioxide levels have been steadily falling. Those are the primary drivers of Earth’s surface temperature. Earth has had five known ice ages, and the one that we are in today is the coldest period in the eon of complex life. Between about 32 million and 25 million years ago, some plants invented a new form of photosynthesis (called C4), in the cooling, drying world of the past 50 million years. That new form conserves carbon dioxide and water, and grasses are the predominant practitioners of C4 photosynthesis. Grasslands have spread since then, and like with flowering plants, another great evolutionary dance began, as animals adapted to grasslands. Browsers became grazers, and grasslands hosted the spectacle of speedy predators that chase grazers, as that dance of predator and prey became supercharged.
When Africa collided with Eurasia, most African fauna lost out to the Eurasian invaders, and apes and the elephant family were the great exceptions. The iconic megafauna of Africa’s grasslands almost all evolved in Eurasia and North America. Until the rise of humans, elephants were the most successful mammals ever. Mammoths evolved to graze all grasslands other than Australia’s, as that continent was too far to swim to. When Earth entered an ice age 2.6 million years ago, it was the first ice age in over 250 million years. Tropical rainforests shrank even further, grasslands expanded, and it surely had profound evolutionary effects on what became the human line. The increasingly sophisticated tools of the human line were critical for survival. Humans are completely dependent on our tools today. We handle tools all day long, from sheets to door knobs to appliances to automobiles. Virtually everything in a home but food, water, and what lives in it is a tool. Civilization is a collection of tools. Without our tools, none of us would survive for long.
There are very few known fossils between chimps and australopiths, such as Ardi, and between chimps and australopiths, the canines shrank, which implies less male fighting over mates, and it is even suggested that pair-bonding in the human line began when the human line became bipedal. The most dramatic change in the human line after stone tools were made was the runaway growth in the human brain. In a little over a million years, the human line’s brain doubled in size. That is likely a unique event in the history of life on Earth. The human brain uses 20% of a body’s energy, and that had to be paid for somehow. The hypotheses range from a shrinking digestive system from better food, increased metabolism, to the reduced energy needed for bipedal locomotion. Homo erectus, which appeared on the evolutionary scene about 2.1 million years ago, was adapted for sleeping on the ground and running, which was a first for a primate.
Today, many in the field call Homo erectus the first member of our genus, but others argue that Homo erectus is not a member of our genus, while others argue that our genus began nearly three million years ago, or that Homo habilis is the first member of our genus. As the story of life on Earth reaches the human chapters, the controversies and debates can become heated, as the findings hit closer to home and the human ego gets engaged. Somebody in my circles recently published this, which was I thought was a pretty good sketch of human psychology.
When those canines shrank, it also likely meant less violence in general. Richard Wrangham found that chimp societies are hundreds of times more aggressive and violent than the most violent human societies are. Azar Gat stated it as succinctly as I have seen: the biological compulsion to survive and reproduce in a world of scarcity is the root of all violence. While the most violent human societies are indeed very violent, they are still less than 1% as aggressive and violent as chimp societies are. Homo erectus must have been far gentler than chimps, although they were still probably far more violent and aggressive than today’s human societies.
Charles Darwin thought that humanity’s two greatest achievements were the control of fire and the invention of language. Today, scientists argue the Homo erectus likely invented both of them, as long as two million years ago. That is quite a change from a generation ago, when both were thought to have been mastered very recently; language maybe only 60,000 years ago, less than a million years ago for fire, and not universally used by the human line until only a few hundred thousand years ago. Those controversies will outlive me.
Not long after those bipedal apes began making stone tools, nearby giant tortoises began going extinct, and they may be among the first casualties of the rise of humans. Armor was a great survival strategy, until something defeated the armor. Crabs and lobsters have massive claws that crack the shells of their mollusk prey, in that arms race.
Homo erectus became an apex predator, and soon after the rise of Homo erectus, Africa’s big animals began going extinct, especially members of the elephant family. Mammoths evolved on the African grasslands and went extinct in Africa with the rise of Homo erectus. But mammoths thrived in Earth’s other grasslands, at least until humans arrived, and then they quickly went extinct.
Around a million years ago, Homo heidelbergensis evolved from Homo erectus, and Homo heidelbergensis is thought to be the direct ancestor of Homo sapiens, as well as of Neanderthals. The tools kept getting more sophisticated, and what is called the Middle Stone Age began about 500,000 years ago (although some consider that it only arrived with Homo sapiens).
About 300,000 years ago, Homo sapiens came onto the evolutionary scene, but early versions had flat skulls like Neanderthals did, and the characteristic globular skulls that humans have today did not appear until about 100,000 years ago, around when they became behaviorally modern, which is likely not a coincidence.
The human journey has always been an interaction of energy, intelligence, and tools, and that will probably never change. In 2007 or so, I began studying what I call the First Epoch of the human journey, which is the time from those first crafted stone tools to the arrival of behaviorally modern humans (likely less than 100,000 years ago). My comprehensive view of the human journey began to form in earnest then. In 2010, when I read a book that was published by an oil company, of all things, my Epochal framework really began to take shape. But it was not until I began writing my essay of 2013-2014 that I finally settled on four Epochs of the human journey so far, and that decision has only made more sense in the years since then. Each Epoch was founded on its energy practices (fire, megafauna, crops, hydrocarbon fuels).
Exactly how Homo sapiens became behaviorally modern and conquered the world has been a vigorously debated scientific subject, and my guess is that a hyper-social dynamic developed in the rising human population in Africa, which allowed for that so-called Great Leap Forward into behavioral modernity. When that happened, nothing could stand in the way of our ancestors. All other human species and the remaining easy meat on Earth were doomed when that happened.