For much of our human evolution, climate played a major part in the success or failure of various human species variants — including the last, homo sapiens — the lone survivor as Chris Stringer so aptly put it in the tile of his 2012 book, “Lone Survivors: How We Came to be the Only Humans on Earth.”

Two years ago, an elegant study in the scientific journal Nature, linked dispersal times and routes out of Africa for archaic and modern humans into the Arabian peninsula, to periods when the climate was favourable — when there was plenty of water, vegetation and game along the routes. 

Dispersion out of Africa occurred roughly every 100,000 years and corresponded to matching dates for fossils belonging to archaic and modern human along the routes and fit with known cyclical changes in the Earth’s orbit about the sun.

Recently, another study in Nature broadened the time period from 400,000 to two million years ago with the goal of correlating habitability — the extent to which temperatures, rainfall and vegetation favoured survival — with the locations and densities of species, such as homo habilis, homo erectus, homo heidelbergensis, neanderthals and modern humans, based on the fossil record and collaborative findings such as tools. 

The scope of the study was gigantic, one measure of which was the basic unit for geographical area — one degree for latitude and longitude, and for time, a thousand years.

For each of those geographic and time units, habitability was correlated with fossil findings for the five species studied.

The geographic area for homo habilis (2.3 to 1.6 mya), the earliest of the homo species, was limited to habitable regions in east and south Africa with no evidence of dispersion out of Africa.

Homo erectus (1.8 to 0.1 mya), the longest lasting of the homo species on record, dispersed widely to habitable regions within Africa, southern Europe, India, East Asia and the southeast islands.

Mary Leakey famously called him dimwitted based on the crudeness of his axes over hundreds of thousands of years.

But, given erectus’ success exploring widely differing regions of the Earth, his story was a surely a success and deserves the name “the traveller.”

Erectus also evolved into heidelbergensis about 900,000 to 800,000 years ago.

With homo heidelbergensis, brain size reached the low limit for modern humans, with commensurate skills fashioning tools and weapons. Like erectus, heidelbergensis has been found in habitable regions of Africa, and throughout Eurasia.

Heidelbergensis was probably ancestral to three main branches — one led to the neanderthals in Europe and to their East Asian cousins, the denisovans, another to a “ghost” species identified by its genome but for which there are no fossils, and the third branch eventually led to homo sapiens in Africa. 

For reasons not entirely clear, neanderthals remained stuck in Europe and the Middle East, although some made it to East Asia, based on genetic evidence that the two cousin species, neanderthals and denisovans, mated.

This study in Nature revealed in graphic illustrations, the amazingly tight correlation between habitability and regions where various homo species settled or spread to and reinforced the strong link between climate and Earth’s orbit about the sun.

That link escaped paleoanthropologists for most of the 20th and early 21st century.   

Modern humans eventually managed to spread to every continent, save Antarctica, and proved to be the most capable of mastering extremes of weather and climate.

That is, until the last several decades when intensive burning of fossil fuels began to have a noticeable impact on mean global temperatures and rising carbon dioxide in the atmosphere and oceans.

The latter two are associated with more severe weather conditions, with the prospect of much worse weather to come if we can’t keep further increases within the United Nations cap of 1.5 C.

Now, deep into a thoroughly nasty war in Ukraine, and  a disastrous conflict in Gaza, the prospect of achieving that minimum goal is receding, as fossil fuel companies crank out more fuel than ever in response to world economies recovering from COVID-19 and dislocations in the supply chain in response to the war, to say nothing of the cost of that war in fossil fuel consumption. 

It’s a mess and reminds me that modern humans, for much of their tenure on Eart,h have been the meanest, most destructive species on record.

And, like lemmings blindly headed to destruction, we are very much on the same path.  

Maybe it’s not too, late but there’s little evidence that humans possess the collective will to rein in armed conflicts and climate change.

The sixth extinction is coming: only, this time, it won’t an errant comet or cataclysmic volcanic eruptions — it will be us.

Despite our many fine achievements, we remain perhaps for the future, fatally tribalistic, and too often treat others we don’t agree with distain and cruelty, on which I rest my case with Ukraine and Gaza.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.