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Wednesday, October 5, 2022
Dr. Brown: Climate, migration and the orbit of the Earth around the sun
Dr. William Brown
Dr. William Brown

Our sun was born in this region of the Milky Way, the third star in the neighbourhood.

The first two long ago completed their cycles from birth to burning brightly before finally collapsing into a dense lifeless remanent, or perhaps exploding in a supernova event, scattering their remains of heavier elements throughout their neighbourhood to possibly be incorporated in nascent stars.

The sun is fated to follow a similar demise in four billion years after expanding into a giant red ball, engulfing Earth and extinguishing all life, before finally collapsing or exploding about the time Andromeda, our closest neighbouring galaxy, is due to merge with the Milky Way.

At least, that’s what astronomers suggest might happen far in the future, well beyond our existence as a species, based on their observations of similar cycles on the grand scale of the universe.

There are, of course, many other cycles of birth, life, death and rebirth. They include thousands of origins stories recounted by Aboriginal peoples and most major religions. and in more modern times by science.

This includes our own origins as a species within Africa at least six million years ago to the emergence of homo species with increasingly larger and capable brains, to the emergence of anatomically and behaviourally modern humans roughly 200,000 years ago.

So far, there’s no evidence that homo’s immediate predecessors, the many versions of the bipedal australopiths with brains the size of modern-day chimpanzees, ever left Africa, But there’s plenty of evidence that homo species’ beginning with homo erectus and later larger-brained and increasingly talented variants, left Africa and dispersed throughout much of Eurasia and beyond.

But there was a weather barrier to overcome. For example, the climate in northeast Africa and northwest Arabia along potential dispersal routes from Africa to Eurasia varied widely from arid periods to lush green periods. The latter occurred roughly every 100,000 when water was abundant in the form of rainfall, rivers and lakes, and a wide variety of animals and plants existed – just what was needed to sustain journeys by small migrant groups of archaic humans such as neanderthals and modern humans.

A 2021 study in "Nature" revealed that those green periods occurred 400,000, 300,000, 200,000, 100,000 and 30,000 to 75,000 years ago and most recently, 55,000 years ago. These periods match archeological records for the presence of dated ancient lakes and rivers, plants, animals and stone tools of increasing sophistication over the last 400,000 years.

Compelling for me was a set of footprints belonging to a modern human who was walking along the shore of an ancient lakebed. The footprints were confidently dated to 110,000 years ago. Amazing, and similar to the footprints of a group of neanderthals walking along a stretch of the south coast of Spain in the same period.

This cyclic period is the series of glacial (think ice age) and interglacial (think global warming) periods which powerfully shape the climate of the Earth. They are related to cyclic changes in the yearly orbital path of the Earth about the sun, which range from a nearly circular path to a more eccentric path with a cycle period of between the two of roughly 100,000 years.

At the peak of the last major glacial period 22,000 years ago, ice covered almost all of Canada, northern U.S. and northern Europe before receding 10,000 years ago to a landscape much like we have now.

The change in the orbit of the Earth is in turn, related to changes in the gravitational effects of other planets in the solar system on our planet, whose orbital paths also change.

Earth’s climate is also related to changes in the tilt of its orbital axis, which follows a 41,000-year cycle, and a phenomenon called precession, which is akin to the wobble of a spinning top. It also affects Earth’s climate, over a cycle period varying from 19,000 to 24,000 years.

As confusing as these influences may seem, their net effect has been to exert a powerful influence on climate change and, of course, the consequences of that in short-term weather changes.

Despite major shifts in global climate most species survive and adapt. What is clear that climate change by creating land-bridges in some areas such as Alaska or favourable dispersal routes in northeast Africa and Saudi Arabia, changed when and how humans and our more ancient ancestral species spread over the planet. For the most part, they thrived, even in tough circumstances.

Astronomy, geology and biology are examples of "deep" history, far deeper than the history I was taught in school or the reach of many sacred texts. I hope that’s changed because perspective comes with long time or "deep time," as some modern historians put it.

In this essay, the clock began a million years following the Big Bang with the creation of the first stars and galaxies and kept ticking until the present. As such, deep time provides a much different perspective on future time.

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.