SCIENCE8 - The History of Human Evolution

I’ve been on a science bent lately, talking about the Moon, the Sun, and the possibility of finding intelligent extraterrestrial life.  Continuing with scientific subjects, this blog is the first of a two-part article about human evolution.  Part 1 will cover what we know about the history of human evolution to the present.  Part 2, in a separate blog, will discuss how humans might evolve in the future.

 

My principal sources for this blog include “Human Evolution,” Britannica.com; “Human evolution,” Wikipedia; “Primate Classification and Evolution,” ck12.org; “How Humans Evolved from our Hominid Predecessors,” humanjourney.as; “Recent Human Evolution, Wikipedia; “Six Changes That Prove Humans Are Still Evolving, With Natural Tweaks to Body,” indiatimes.com; and numerous other online sources.

Introduction

Human evolution is the process by which human beings developed on Earth from now-extinct primates.  This process involved the gradual change to the DNA of primate species over many generations, and development of traits such as erect posture, bipedal locomotion (walking on two feet), larger brains, behavioral characteristics such as specialized tool use, and, communication through language.

The basic mechanism of human evolution is (Charles Darwin’s) natural selection, where genetic mutations that are beneficial to an individual's survival are passed on through reproduction.  This results in a new generation of individuals that are more likely to survive to reproduce.  Gradually, these mutations and their associated traits become more common among the whole group.  Natural selection can also give rise to a new and distinctly different species.

Charles Darwin was an English naturalist, geologist and biologist, best known for his contributions to evolutionary biology.  Darwin published his theory of evolution with compelling evidence in his 1859 book On the Origin of Species.  Today, natural selection is accepted as the basic mechanism of evolution.

Charles Darwin (1809-1882), with his natural selection theory of evolution, was one of the most influential persons in human history.

The main source of knowledge about the human evolutionary process has traditionally been the fossil record, but since the development of genetics beginning in the 1970s, DNA analysis has come to occupy a place of comparable importance.

Based on accumulated knowledge to date, humans very likely first evolved in Africa about 315,000 years ago.

I’ll continue with a discussion of the early evolution of primates - through the development of the first humans, followed by the evolution of early humans to today’s modern human.  Then, I’ll step back and review the anatomical changes that occurred in humans along the evolutionary path.  I’ll finished with a snapshot of recent and ongoing human evolution.

 

Early Evolution of Primates

Genetic studies show that primates evolved about 85 million years ago from small mammalian ancestors that lived in the trees of tropical forests.  Many primate characteristics represent adaptations to life in this challenging, three-dimensional tree-living environment.

Primates have a skeletal structure that allows a wide range of motion of the limbs.  This is reflected in the diversity of ways that they move around.  They may leap, climb, swing from branch to branch, walk on all four legs, or walk on two legs.  Their relatively unspecialized extremities, with long digits and nails (instead of claws), are capable of many different functions - unlike, for example, the more specialized clawed feet of cats or dogs.

Primates have traditionally been divided into two major groups, called prosimians and anthropoids. Prosimians include lemurs, lorises, pottos, and tarsiers.  Anthropoids include monkeys, gibbons, orangutans, gorillas, chimpanzees, and humans.  Prosimians are generally small in size and have characteristics similar to the earliest primates, whereas anthropoids tend to be larger and less similar to early primates.

Primates can be classified on the basis of their evolutionary history. The chart below shows the common ancestors of primates.  For comparison, the traditional classification of primates into prosimians and anthropoids is included at the top of the evolutionary tree.

Common ancestor evolutionary tree for primates.

 

If you look at the left side of the tree above, you can see approximately how long ago the different groups of primates diverged from their common ancestors.

An ancestral primate relative left abundant fossil remains in North America and Europe that date to 55-58 million years ago, but probably originated earlier.  This primate relative was a small, at least partly tree-living mammal, that may have eaten fruits and leaves.  This now-extinct relative had a small brain and claws instead of nails.  Its eyes were located on the sides of the head, which meant it lacked three dimensional vision.  With these traits, this relative itself was not a primate, but it was a likely ancestor of primates.

Once the first primates emerged, they continued to evolve until the present.  During that long time span, primates underwent a series of evolutionary divergences, in which the major groups of primates branched off and new common ancestors emerged.

The first major divergence probably occurred close to 60 million years ago, when the lineage that would evolve into modern lemurs, lorises, and pottos branched off from other early primates.

1.       The divergence that led to the tarsiers probably dates to about 55 million years ago. The earliest tarsier-like fossils date to that time in East Asia.

2.       The emergence of the common ancestor of all anthropoid primates (monkeys, gibbons, gorillas, chimpanzees, and humans) is thought to have occurred around 45 million years ago.

3.       The group of anthropoids that would evolve into modern New World monkeys probably split from their African ancestors at about the same time.  They traveled from Africa to South America across the Atlantic Ocean, which, due to continental drift, was much narrower then.  Most likely, a few primates were carried out to sea on a natural raft of vegetation, and were then carried to the New World by currents.

4.       A possible common ancestor of Old World monkeys, gibbons, gorillas, chimpanzees, and humans lived around 35-33 million years ago.  It was about 22 to 36 inches in length and weighed about 17.7 pounds, and lived in subtropical forests, where it was most likely a tree-living quadruped that fed on fruit.

5.       A possible common ancestor of gibbons, gorillas, chimpanzees, and humans lived about 23-14 million years ago; probably it walked on four limbs and fed mainly on fruit, and had a brain larger than a modern monkey.

6.       Ancestors of gorillas diverged in Africa about 8 million years ago.  Approximately 7-6 million years ago in Africa, the chimpanzee and human lineages finally diverged.  Our first true human ancestors began to evolve then.

Based on the common-ancestor tree, humans share the most recent common ancestor with chimpanzees.

The main evolutionary trend of primates has been the elaboration of the brain.  Comparative studies show a continual trend toward higher intelligence going from prosimians to New World monkeys, to Old World monkeys, and finally to humans.  The part of the brain that has increased the most is involved with sensory perception, generation of motor commands, spatial reasoning, and conscious thought.  While other mammals rely heavily on their sense of smell, the tree-living life of primates has led to a tactile (touch) and visually-dominant sensory system.  At the same time, there has been a reduction in the olfactory (smell-related) parts of the brain.

With their large brains, primates have advanced cognitive abilities.  One indication of this is their tool use.  Besides humans, many other species of primates, especially anthropoids, have been observed using items in their environment as tools.  Gorillas have been observed using a stick to test the depth of water as they wade through it.  Monkeys have been documented cracking nuts by placing them on an "anvil" stone and hitting them with another large stone.  Monkeys also use stones to crack open crabs and other shellfish.  Some primates make tools, as well.  Chimpanzees, for example, strip leaves from twigs and insert the twigs into termite mounds to "fish" for edible insects.  They also sharpen sticks to use as spears for hunting small mammals.

Another trend in primate evolution has been an increasing dependence on complex social behavior. Primates are among the most social of animals.  They live in mated pairs, small family groups, or groups of up to dozens of individuals.  Cooperation within the group helps to ensure that all group members survive.  Cooperative social behaviors include grooming, food sharing, and collective defense against predators or of territory.

Evolution of Humans

We humans are members of a primate species called Homo sapiens (“wise man” in Latin) that very likely first evolved in Africa about 315,000 years ago.  We are now the only living members of what many zoologists refer to as the human family, but there is abundant fossil evidence to indicate that we were preceded for millions of years by other primates in the human family (called hominins).

The evolution chart below shows the divergence of gorillas from hominins, chimpanzees, and bonobos; the divergence of hominins from chimpanzees and bonobos; and a representative evolution of hominins to Homo sapiens (modern humans).

Bonobos, once known as pygmy chimpanzees, and thought to be a subspecies of chimpanzees, have been recognized as a distinct species.

 

The divergence and evolution of humans from common ancestors.

The earliest hominins arose some 6 to 7 million years ago.  (At least seven primate species have been advocated as possible direct ancestors to the early hominins, though there is no consensus among experts.) 

Referring to the above evolution diagram, eight primate species are shown - each eventually going extinct - along the evolution path to Homo sapiens.  (Other primate species certainly existed; the hominin species shown have consensus agreement among experts.)

The first hominin fossils found, remains of Homo neanderthalensis, were found in Germany in 1856, three years before the publication of Charles Darwin’s, On the Origin of the Species.  Despite the 1891 discovery of Homo erectus fossils at Trinil, Java, it was only in the 1920s when such fossils were discovered in Africa, that fossils of hominin species began to accumulate.  During the 1960s and 1970s, hundreds of fossils were found in East Africa by the Leakey family in the regions of the Olduvai Gorge and Lake Turkana.  These finds cemented Africa as the “cradle of humankind.”  In the late 1970s and the 1980s, Ethiopia emerged as the new hot spot of paleoanthropology.  Meanwhile, discoveries of additional hominin fossils, from several different species, were accumulating around the world, including northwestern Africa, south Africa, Europe, southern Asia, Indonesia, and Australia.

 

Sites of important hominin excavations.

The trove of fossils from Africa and Eurasia indicates that, unlike today, more than one species of the human family lived at the same time for most of human history.

Prime examples are Homo erectus and Homo neanderthalensis - both arising in Eurasia.  Homo erectus (upright man) first emerged about 2 million years ago, died out about 100,000 years ago, and was the first human ancestor to spread to northern Africa and southern Asia.  Homo erectus was the first human species to exhibit a flat face, prominent nose, and possibly sparse body hair coverage.  Homo erectus developed predatory behavior and coordinated hunting.  Homo erectus is also postulated to have been the earliest human ancestor capable of using fire, hunting and gathering in coordinated groups, caring for injured or sick group members, and possibly seafaring.

Homo neanderthalensis (named after the German valley where the first fossils were found) emerged at least 200,000 years ago and died out perhaps between 35,000 and 24,000 years ago, living in Europe and western Asia.  They manufactured and used tools (including blades, awls, and sharpening instruments), developed a spoken language, and developed a rich culture that involved hearth construction, traditional medicine, and the burial of their dead.  Neanderthals also created art; evidence shows that some painted with naturally occurring pigments.  Neanderthals were replaced by modern humans (Homo sapiens).

The linear view of human evolution (represented by a branching tree) began to be abandoned in the 1970s as different species of humans were discovered that made the linear concept increasingly unlikely.  In 2010, evidence based on molecular biology was published, revealing unambiguous examples of interbreeding between archaic and modern humans.  These discoveries show that human evolution should not be seen as a simple linear or branched direct progression, but a mix of related species.  In fact, genomic research has shown that interbreeding between substantially diverged lineages is the rule, not the exception, in human evolution.   Furthermore, it is argued that interbreeding was an essential creative force in the emergence of modern humans.

Homo sapiens are distinguished by a more highly developed brain that allows for articulate speech and abstract reasoning.  Humans display a marked erectness of body carriage that frees the hands for use as manipulative members.  The oldest known remains of Homo sapiens - a collection of skull fragments, a complete jawbone, and stone tools - date to about 315,000 years ago.

In 2017, the announcement of this discovery in Jebel Irhoud, Morocco rocked the human evolution world.  Until then, the firmest finds of early Homo sapiens were from two sites in Ethiopia, dating to around 200,000 years old.  Those fossils put the spotlight on eastern sub-Saharan Africa for the origin of our species.  But the new discovery in northwestern Africa, more than 3,000 miles away, set the origin back over 100,000 years and widened the “cradle of mankind” to encompass all of Africa! 

Evolution of the human family.

 

Modern humans started burying their dead, using animal hides to make clothing, hunting with more sophisticated techniques (such as using trapping pits or driving animals off cliffs), and engaging in cave painting.  Artifacts such as fish hooks, buttons, and bone needles show signs of variation among different populations of humans, something that had not been seen in human cultures prior to 50,000 years ago.

Among concrete examples of modern human behavior, anthropologists include specialization of tools, use of jewelry and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks.

Homo sapiens dispersed from Africa in several waves, from possibly as early as 200,000 years ago, and certainly by 150,000 years ago. leading to the lasting colonization of Eurasia by 45,000 years ago and Oceania by 50,000 years ago.  Dispersal continued into northern Asia about 20,000 years ago, and into North America about 15,000 years ago.

Modern human (Homo sapiens) migration populated the world.  (Numbers in the chart are “years ago” dispersals.)

 

Anthropologists and biologists everywhere agree that we and the other hominids, both living and extinct, are somehow related.  Yet the exact nature of our evolutionary relationships is still the subject of debate and investigation. 

Anatomical Changes

Human evolution, from its first separation from the last common ancestor of humans, chimpanzees, and bonobos, is characterized by a number of changes.  The most significant of these adaptations are bipedalism, increased brain size, sexual differences, and thumb-pinky opposition.  The relationship between these changes is the subject of ongoing debate.

Bipedalism.  Bipedalism is the basic adaptation of gorillas, chimpanzees, bonobos, and humans (hominid family).  It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long-distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun.

Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet, and ankles.  The femur evolved into a slightly more angular position to move the body’s center of gravity toward the geometric center.  The knee and ankle joints became increasingly robust to better support increased weight.  To support more weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider.  The big toe moved into alignment with the other toes to help in forward locomotion.  The arms and forearms shortened relative to the legs, making it easier to run.

The most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking.  Bipedal hominids have a shorter, but broader, bowl-like pelvis due to this.  A drawback is that the birth canal of bipedal hominids is smaller than in knuckle-walking hominids.

The shortening of the pelvis and smaller birth canal had significant effects on the process of human birth, which is much more difficult in modern humans than in other primates.  During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal, and rotate about 90 degrees upon exit.  The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period, leading to the relative immaturity of human offspring, who are unable to walk much before 12 months, and have slower development, compared to other primates, who are mobile at a much earlier age. 

The increased brain growth after birth, and the increased dependency of children on mothers, had a major effect upon the female reproductive cycle, and the more frequent appearance of non-descendent parenting in humans when compared with other hominids.  Delayed human sexual maturity also led to the evolution of menopause, with one explanation being that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more children of their own.

Increase in Brain Size.  The human species eventually developed a much larger brain than that of other primates - in modern humans, nearly three times the size of a chimpanzee or gorilla brain.  This brain increase, manifested during postnatal brain growth, allowed for extended periods of social learning and language acquisition in juvenile humans.

Sexual Differences.  Reduced differences in size or appearance between the sexes is visible primarily in the reduction of the male canine tooth relative to other hominids (except gibbons), and reduced brow ridges and general robustness of males. 

Another important physiological change related to sexuality in humans was the evolution to continuous female sexual receptivity.  Humans are the only hominids in which the female is fertile year-round, and in which no special signals of fertility are produced by the body, such as genital swelling or biology-caused seeking of the male’s attention by females.

Nonetheless, humans retain a degree of sexual differences in the distribution of body hair and subcutaneous fat, and in overall size, males being around 15% larger than females.

Thumb-Pinky Opposition.  The contact between the thumb and the tip of the little finger of the same hand - is unique to the Homo family.  In other primates, the thumb is short and unable to touch the little finger.  This ulnar opposition facilitates the precision grip and power grip of the human hand, underlying all the skilled manipulations.

Other Changes.  A number of other changes have also characterized the evolution of humans, among them an increased importance on vision, rather than smell; a smaller gut; faster basal metabolism (the rate at which the body uses energy while at rest to maintain vital functions such as breathing and keeping warm); loss of body hair; evolution of sweat glands; development of a chin (found in Homo sapiens alone); development of the styloid process (a slender pointed piece of bone just below the ear, projecting down and forward from the skull to serve as an anchor point for several muscles associated with the tongue and larynx); and the development of a descended larynx (housed in the thyroid cartilage for protection of the trachea and sound production).

In summary, humans share a common ancestor with gorillas, chimpanzees, and bonobos that lived about 10 million years ago, when gorillas diverged into a separate line.  About 7 million years ago, the human line diverged from chimpanzees and bonobos, who are our closest relatives.

Anatomical differences between humans and chimpanzees, our closet relative.

 

Recent and Ongoing Evolution

Genetic studies have demonstrated that modern humans are still undergoing natural selection for several traits.  Some of these are due to specific environmental pressures, while others are related to lifestyle changes since the development of agriculture (10,000 years ago), urbanization (5,000 years ago), and industrialization (250 years ago).  It has been argued that human evolution has even accelerated since these developments.  Here are a few examples.

Survivors of infectious disease outbreaks drive natural selection by giving their genetic resistance to offspring.  Our DNA shows evidence for recent selection for resistance of killer diseases like Lassa fever and malaria.  Selection in response to malaria is still ongoing in regions where the disease remains common.

Mutations allowing humans to live at high altitudes have become more common in populations in Tibet, Ethiopia, and the Andes.  The spread of genetic mutations in Tibet is possibly the fastest evolutionary change in humans, occurring over the last 3,000 years.  This rapid surge in frequency of a mutated gene that increases blood oxygen content gives locals a survival advantage in higher altitudes, resulting in more surviving children.

Diet is another source for adaptations.  DNA evidence shows a recent adaptation that allows Eskimos to thrive on their fat-rich diet of Arctic mammals.  Studies also show that natural selection favoring a mutation allowing adults to produce lactase - the enzyme that breaks down milk sugars - is why some groups of people can digest milk after weaning.  In 2016, scientists discovered a mutation in the gene among vegetarians in India which allowed them to produce from vegetarian sources, essential omega-3 and omega-6 fatty acids, responsible for healthy brain function.

We may well be adapting to unhealthy diets too.  One study of family genetic changes in the U.S. during the 20^th century found selection for reduced blood pressure and cholesterol levels, both of which can be lethally raised by modern diets.

Finally, our bones are getting lighter.  Our ancestors had denser and heavier bones than us.  A study from 2015, undertaken by an anthropologist Habiba, posited that our skeletons have become more delicate as food security improved with agriculture - another proof of how we evolve as a species is dictated by how our societies and humanity's activity levels evolve.

Where does Human Evolution go from Here?

Nowadays, with the availability of better healthcare, food, heating and hygiene, the number of “hazards” we experience in our lives has dramatically reduced.  In scientific terms, these hazards are referred to as selection pressures.  They put pressure on us to adapt in order to survive the environment we are in, and reproduce.  It is selection pressure that drives natural selection (survival of the fittest) and it is how we evolved into the species we are today.

When you ask for opinions about what future humans might look like, you typically get one of two answers.  Some people trot out the old science-fiction vision of a big-brained human with a high forehead and higher intellect.  Others say humans are no longer evolving physically - that technology has put an end to the brutal logic of natural selection and that evolution is now purely cultural.  These people say, “It’s not really a biological question anymore, it’s technological.”

In Part 2 of this article, “The Future of Human Evolution,” I’ll talk about the transition from natural evolution to technological evolution.