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From primates to humans, follow the evolutionary path

May 31, 2013

Different skull representations of evolutionary stages species went through as they mutated between the primate and human species. On the left is a modern human skull, while on the far right is that of a chimpanzee. (Photo by Shaun Kirby)

ASHAWAY—Sitting on a rainy afternoon at the Ashaway Free Library, the opportunity to learn about the evolution of humans might not be expected. But this past Saturday, University of Rhode Island Professor Holly Dunsworth guided a small group of local listeners through the history of modern humans, from our shared ancestry with other primates such as apes and chimpanzees to our development of bipedalism and an enlarged brain.

Dunsworth, a paleoanthropologist by trade, began with the biological reasoning behind why, approximately seven million years ago in Africa, the human lineage began to split from that of modern day chimpanzees, stating that over such a great period of time, reproduction over generations created ‘mutations’ which eventually lead to the evolution of Homo sapiens sapiens, or us.
“We all have different genomes from our parents, not just because we have a combination of their genomes, but we actually have new mutations,” said Dunsworth. “Geneticists can take our genome up against [that of] a chimpanzee, count the differences, and use the clockwork rate of time to see how far back we split off and stopped sharing genes.”
“Chimpanzees, when you look at their genome, share more with us than they do with gorillas, which is how we know that we are most closely related,” she continued. “[Humans] have experienced a faster rate of evolution than chimpanzees and other primates, so we are really weird, but that doesn’t mean that we are somehow more evolved.”
Biological change is and always has occurred throughout time, said Dunsworth, although the difficulty of understanding human evolution rests in our scientific and terminological limitations. Dunsworth noted our current issue with grasping what scientists’ call ‘deep time,’ or the vast swath of years it has taken us to evolve into the so-called top of the world species we are today.
“Deep time is something everyone has struggled with, and to say we are all related and have one family history is an impossible thing to imagine,” said Dunsworth. “We are here for a geological instant, so [understanding evolution] isn’t something you can overcome unless you are constantly aware of this limitation of our imaginations.”
“You can appreciate deep time if you go to the Grand Canyon,” she added.
Dunsworth further explained that our classification of species throughout their evolutionary history is arbitrary, which presents further issues in studying a process which has occurred gradually over thousands and millions of years.
“The biological species concept goes, ‘if you can’t make babies together, you are separate species,’ but there are exceptions,” said Dunsworth. “These species are great for talking about biology, but ultimately arbitrary.”
“If we go back far enough in time we may share a common ancestor [with chimpanzees], but no where at any point did a mother give birth to a new species,” she continued. “So if everything that ever lived on the Earth were in this room right now, we would have a hard time agreeing how to box them up because the spectrum of variation would be so gradual.”
Nevertheless, scientists such as Dunsworth have strived to gain a firmer grasp on how humans evolved from their primate brethren, none more popular than Charles Darwin, whose scientific theory of evolution is the foundational work in understanding how earth’s various species have derived from common ancestors.
Dunsworth, whose main research on human evolution has been conducted either through DNA studies or sniffing out the fossil record of early humans in Kenya, has dusted off and come into contact the bones of our earliest ancestors, such as the approximately 5 million year old Ardipithecus or her descendant, Australopithecus afarensis, found by paleoanthropologists in Ethiopia in 1974 and more commonly known as ‘Lucy.’
“Lucy is great evidence and we are all sure she is a related ancestor,” said Dunsworth. “Lucy was probably 3.5 feet tall, but she was tiny for her species, and we think the males were really big, twice her size, we also have footprints of Lucy’s species which are much more like ours than a chimpanzee’s.”
These early human ancestors then began to evolve gradually from walking on all limbs to just two, and as bipedalism became a more prevalent genetic mutation among subsequent generations, brain size also began to increase.
“The big brain story is what everyone likes to ponder, but the bipedalism story is equally important,” said Dunsworth. “We can track it in the fossil record, and it happened fairly recently. Our skeleton metamorphosed during the first four million years, and now we have a more flexible spine and a totally different pelvis [than our biological ancestors], which helps us balance because we spend a lot of time on one foot.”
“In the last two million years is when the brain starts to explode in size, so I don’t think it is any coincidence that the body comes first then the brain,” she continued. “For the longest time, people thought that big brains preceded bipedalism. This is how culture can bias science.”
As our ancestors’ biology changed genetically, their capabilities in their surrounding environment began to improve. The Nariokotome Boy, for example, was an approximately eight year old member of the homo erectus species whose bone structure is more decidedly closer to modern humans. He was a hunter, and a good one at that, having the physical capability to run long distances and throw farther.
“For the last two million years, we have an upright, bipedal body, and this is where we start to find the first stone tools,” explained Dunsworth. “At about 600,000 years ago, we have the earliest evidence for fire. Although homo erectus started about two million years ago, we don’t have evidence that they could control fire until a million years into their evolutionary history, and not until about 400,000 years ago did [Neanderthals] use it regularly.”
After homo erectus, we come to Neanderthals and then modern humans. All throughout the biological evolutionary process, these now-extinct human species began to spread out into first Asia, Australia and the Americas, then lastly into Europe as they gradually searched for new hunting grounds or social factors forced them from their original communities.
The journey was long, but Dunsworth hopes that scientists and the public alike can look back and appreciate the vast amounts of changes our ancestors experienced, even though they are all now extinct, culminating in the emergence modern humans.
“Neanderthals have this bad rap of being klutzes, for example, because they have all these injuries,” said Dunsworth. “We think it is because they were ambush hunters and weren’t doing action at a distance, throwing spears and debilitating prey with rocks and spears [like their human descendants]. We think they were jumping on wooly mammoths, and we know this because their injuries mirror those of rodeo riders.”
“Whatever [species] it is, us, chimpanzees, dogs or fish, we are not limited to learning about ourselves by looking just at ourselves,” she emphasized. “It is a beautiful perspective to look at other things and appreciate how we came to be.”

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