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Monday, May 20, 2024
Dr. Brown: Was Lucy an ancestor of modern-day humans?
This reconstruction of Lucy is from the National Museum of Anthropology in Mexico. SOURCED

The evolution to where we are now as a species began five to six million years ago, with the last common ancestor to what would eventually become modern humans emerging 150 to 200 million years ago, and the other branch that led to chimpanzees and, later, bonobos.

Between then and now, there were extensive adaptions, the most important of which, overall, was the transition to full bipedalism and the associated suite of adaptive changes in the feet, especially the great toes, ankles, knees, hips, pelvis, spine, and an upright position of the foramen magnum at the base of the skull to support what would become an upright head with a forward-facing face.

Taken together, those changes favoured a stable upright stance and fluid, well-balanced walking and running, while freeing the forelimbs for more precise manipulative skills such as making and using tools, and eventually doubling — even tripling — the size of the brain, linked to organizational changes to the nervous system, to support bipedalism and repurposing the hands and arms.

There were other changes, too, in the jaw and teeth that reflected dietary changes, and to the brain, to favour greater specialization between the hemispheres to reflect increased dominance of one hemisphere over the other for different functions such as speech, emotional control and cognitive functions.

Many functionally related adaptations tended to evolve together, given for example, that the spine, pelvis, hip, knee, ankle and feet function as a unit.

Along the way, the evolutionary paths toward more highly developed species were often marked by the appearance of several closely related species who might have shared the same territory and time period, but not necessarily similar dietary habits or behaviours.

Looking back at the fossil record, the broad trends — bipedalism, increasingly dextrous arms and hands and bigger, more asymmetrical brains — stand out.

What’s not so clear is what specific roles were played by the 20, so far, identifiable interim species in the emergence of modern humans — that is, which species were favoured most by chance and climates?

Climate affects vegetation, food resources, water supply and temperature and there were many climate shifts in the six million years since the last common ancestor, each which would have favoured the evolution of specific functional, behavioural and anatomical features and could change in response to changes in climate.

Of the 20 or more identifiable species variants, three stand out: Lucy, neanderthals and modern humans.

Why Lucy?

Because she’s the one australopith (Australopithecus afarensis) who caught the world’s attention when she was found in Ethiopia with the most complete and oldest skeleton to that date.

She was discovered by Donald Johanson and his student, Tom Gray, in Ethiopia in 1974 and was given the name Lucy after the Beatles’ song, “Lucy in the Sky with Diamonds.”

Overnight, she became a sensation, viewed by human celebrities including former U.S. president Barack Obama, and placed in the direct line toward modern humans.

She was dated to 3.18 million years ago and despite her kind’s bipedal walk, her brain was chimpanzee-sized.

That was 70 years ago. Since then, other australopiths with differing features and time periods, but all variants on an australopith model, have been discovered, muddying the evolutionary lines to modern humans.

Skipping ahead roughly 2.5 million years in Africa, three lines diverged 600,000 years ago from a species with an almost human-sized brain (late erectus I call him): one line led to what became neanderthals in Eurasia 400,000 years ago and in Asia to the denisovans; another line led eventually to modern humans 200,000 years ago; and, a third line to a ghost species for which we have genetic evidence, but no fossil evidence so far.

Throughout the long evolution of hominin species, several species often coexisted at the same time if not place.

For example, 100,000 years ago, neanderthals, denisovans, modern humans, late bigger-brained versions of erectus and Pygmy-sized versions existed in at least three separate locations.

Now, as Chris Stringer wrote in his wonderful book “Lone Survivor,” only one species remains.

Of all the species variants who preceded us, by far, we have been the cleverest and most destructive of all — killing off large animals on every continent we entered and continue to kill in Ukraine, the Middle East, Lebanon and assorted places and a wide variety animal and plant species these days.

That’s a short evolutionary picture of human evolution and certainly not the straight-line evolution popularized not so long ago.

Our evolution by natural selection and chance was far messier and complicated affair, which given repeated patterns of migration, divergence and convergence, and sex between different species, would be what we might expect.

What’s obvious is that the fossil and genetic records are very incomplete.

If that were not enough drama, our prehuman ancestors 900,000 years ago almost went extinct during a prolonged intense glacial period.

They, and therefore we, survived. I, for one, will continue to marvel at Lucy: she’s the star, if not the central character, in the story of human evolution.

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.  

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