human an gorilla skeletons
What distinguishes an ape from a monkey is the fact that apes don’t have a tail. Humans are apes. They are just as hairy as other apes, but their hair is shorter and finer. Apes are regarded as more intelligent than monkeys. They have rapid eye movement and may dream. They can recognize themselves in a mirror while monkeys think they are confronted with another monkey. Apes and humans are the only creatures that have spindle cells — large cigar-shaped cells neurons linked with emotion, problem-solving, a moral sense and a feeling of free will — in their brains.

According to Merriam Webster dictionary a hominid is “any of a family (Hominidae) of erect bipedal primate mammals that includes recent humans together with extinct ancestral and related forms and in some recent classifications the gorilla, chimpanzee, and orangutan.” According to Wikipedia: “The term “hominid” is also used in the more restricted sense as hominins or 'humans and relatives of humans closer than chimpanzees'. In this usage, all hominid species other than Homo sapiens are extinct.”

Research by geneticists in the mid 2000s determined that the human genome and chimpanzees are only different by 1.23 percent. The one small percentage difference encompasses 35 million individual chemical changes accumulated over the 5 million to 7 million years during which the species evolved apart. Put another way humans and chimpanzees share 98.77 percent of the same genetic material. Not everyone likes this figure. A study in Japan however that 15 percent of genes of humans and chimpanzees are different.

A poll found that 51 percent of Americans felt that primates should have the same rights as human children. Apes and monkeys are though to do alright in captivity if they are given some of their own kind to hang out with and enclosure that in into considerations their social and intellectual needs.

First Primates and Apes

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Sahelanthropus tchadensis, the first hominid?

The oldest primate fossils date to around 55 million years ago. There is circumstantial evidence based on mathematics and probability that they lived as far back as 80 million years which would have made them contemporaries of the dinosaurs.

The earliest human ancestor so far discovered is a 33 million year old arboreal animal nicknamed the "dawn ape" found in the Egypt's Faiyum Depression. This fruit-eating creature weighed about eight pounds (three kilograms) and had a lemur-like nose, monkey-like limbs and the same number of teeth (32) as apes and modern man.

About 25 million years ago the line that would eventually lead to apes split from the old world monkeys. Between 20 million and 14 million years ago orangutans split off from the other “great apes,” chimpanzees, gorillas and humans. Between 14 million and 5 million years ago numerous species of early apes spread across Asia, Europe and Africa.

Between 11 million and 8 million years ago gorillas split off from chimpanzees and humans. In 2007 an Ethiopian and Japanese team of scientists announced in a Nature article that they had found a 10-million-year-old ape 170 kilometers south of Addis Ababa in Ethiopia that was very similar to a gorilla that provided some grounds for pushing back the date of the split between humans and apes. The discovery also supported the theory that the apes that spawn humans originated in Africa.

Fossils of 20.6 million-year-old common ancestor of man and apes was unearthed in Uganda in the 1960s. The animals was about 1.2 meters tall and weighed between 40 and 50 kilograms and was described by thought as a "cautious climber."

Baboon-size apes that lived in East Africa about 15 million years ago may have spent most of its time on the ground. This finding is based on the hand, finger arm and shoulder bones from a ape called Equatorious found in 1993 in the Tugen Hills of north central Kenya. A fossil of an ape that lived 13 million years ago in Spain has been described as a common ancestor to all apes and hominids and led some to theorize that the ancestral ape that spawned gorillas, chimpanzees and human came to Africa from Eurasia.

Rudapithecus is the name given to a 10-million-year-old great ape unearthed in Hungary. Some have called the ape the closest fossil hunters gave come to finding a common ancestor of humans and African apes. Named after the village of Hungarian Rudabanya, near where it was found, it had a body and brain about the size of a chimpanzee. Its long arms and curved fingers indicate it spent a lot of time hanging from the branches of trees. Modest-size molars and thin tooth enamel suggested it ate soft fruits.

In 2011, Reuters reported: Ugandan and French scientists had discovered a fossil of a skull of a tree-climbing ape from about 20 million years ago in Uganda's Karamoja region. The scientists discovered the remains in July while looking for fossils in the remnants of an extinct volcano in Karamoja, a semi-arid region in Uganda's northeastern corner. "This is the first time that the complete skull of an ape of this age has been found. It is a highly important fossil," Martin Pickford, a paleontologist from the College de France in Paris, said. [Source: Elias Biryabarema, Reuters, August 2, 2011]

Pickford said preliminary studies of the fossil showed that the tree-climbing herbivore, roughly 10-years-old when it died, had a head the size of a chimpanzee's but a brain the size of a baboon's, a bigger ape. Bridgette Senut, a professor at the Musee National d'Histoire Naturelle, said that the remains would be taken to Paris to be x-rayed and documented before being returned to Uganda. Uganda's junior minister for tourism, wildlife and heritage said the skull was a remote cousin of the Hominidea Fossil Ape.

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Split Between Apes and Hominids

Based on DNA evidence in blood proteins, molecular biologists guess that the hominid line split off from the ape line between 5 and 8 million years ago, a period of time in which little is known about apes or hominids and there is little data in the fossil record.

The generally accepted assumption is that gorillas and ancestors of chimpanzees and humans split around 6 million to 8 million years ago. Some DNA evidence seems to indicate that hominids and chimpanzees split between 5.5 million and 6.5 million years ago.

Calculations made by geneticists based on the differences between genomes indicates that the chimpanzees and hominids diverged no later than 6.3 million years ago and probably earlier than 5.4 million years ago. This finding raises questions about fossils that are more than five million years old — namely Sahelanthropus tchadensis and Orrorin tugenensis — that are claimed to belong to hominid species.

A team led by David Reich of the Broad Institute in Cambridge, Massachusetts has suggested that one explanations for the discrepancy between the fossil record (that says the oldest hominid are 7 million years old) and genetic evidence (which dates hominid and chimpanzee divergence at around 5.3 million years ago) is that chimpanzees and early hominid might have had sex with each other and interbred. That could also explain why some early hominids have strange mixture of human and chimpanzee traits. One should not jump to too many conclusions though as there is still a lot of uncertainty over dating and inferences made from small quantities of fossils.


Primates, Hominids and Bipedalism

Bipedalism (moving on two legs) is one of the key characteristics that defines hominids and humans. A hominid after all is defined as a creature that stands upright and walks and runs primarily on two legs. It is thought that bipedalism developed in hominids between 4 million and 8 million years ago. No one is sure how or why some apes, who were for the most part arboreal at that time, dropped out of the trees and began walking upright. A popular theory is that it first evolved to free the hands to carry food.

Of more than 250 species of primates, only one — humans — walks around on two legs. Walking is an activity in which the legs move the body forward and act like pendulums balancing the body as it moves along. Key to this movement is: 1) the ability to fully extend the knees to create the pendulum effect; 2) the forward curve of the back and the inward sloping of the thighbones to create a center of gravity over the feet; and 3) the gluteal abductor muscles attached to the pelvis that keeps the body from toppling over sideways when our weight is on a single foot.

Developing these features took some time and its sometimes hard to imagine how it could happen in a step-by-step evolutionary fashion without creating a bunch of awkward beasts that were neither very good at climbing trees or walking. Running is thought to have developed about 2 million years ago. It is a totally different activity than walking. It is more of a pogo-stick-like motion using tendons in the legs as elastic springs.

Bipedalism is regarded as the first major step in the development of our human ancestors. The second major step is toolmaking, which occurred about 2.6 million years ago. In addition to creating a more efficient locomotion mechanism, bipedalism also freed the hands to do other things such as develop tools. These developments in turn lead to the formation of a larger brain, the main thing that separates modern man from apes today. Bipedalism saved energy and water. It required less energy knuckle-walking. An erect body is higher off the hot ground and has less area exposed to the sun, creating the need for cooling water. Bipedalism also made it easier to reach fruit and berries off of low bushes in grasslands and stand up and look for predators on the horizon.

chimp and human thighbones

Bipedal Body Features

Humans have a long curved spine, femurs (thighbones) that angle inward to pushed the center of gravity of the torso over the feet while chimpanzees have a short and stiff spine and straight femurs. Humans also have a broader pelvis and specialized hip joints and muscle that prevent them from swaying like chimps when they walk on two feet. Human spines are shaped like an “S” which is an effective design for maintaining balance, assisting locomotion and bearing the weight of the upper body.

Chimpanzees can not lock their legs straight or extend their knees. Instead the have to use muscles to support their body weight when they stand and energy is expended rocking back and forth when they walk. Two-legged walking is a temporary, transitory method of mobility for them.

Humans also have concave socket-like condyles (rounded prominence at the end of a bone, apes are convex), tibias with flared buttresses, femurs designed for carrying more weight, and bone enlargements that act like shock absorbers. All these help humans walk and support their weight. The human foot has an arch and aligned toes and a large, puffy heel bone with lattice-like structure and covered with paper-thin layers of bone — all of which are conducive walking. Chimpanzees have a thumb-like big toe that is useful for grasping and is an impediment to walking.

Thomas Greiner, a physical anthropologist at the New York Chiropractic College, has suggested that the fact that humans have bigger butts than apes may have played a big role in effectively walking upright. Apes have small butt muscles and can't stay upright for long. The large human butt muscle provide stability for walking or standing.

In a study published in the Proceedings of the National Academy of Sciences, researchers concluded that humans use one forth the energy walking on two legs that chimpanzee use when knuckle-walking based on how humans and chimpanzees perform on treadmill test. This finding provides evidence to the theory that early hominids began engaging in bipedalism because it is more efficient.

First Hominids, Sex, Motherhood, Big Toes and the Costs of Bipedalism

pelvis in apes and man
Scientists estimate that around six million years ago, female hominids switched from being sexually receptive during brief periods of estrus to being sexually receptive year round. Scientists estimate this change took place around the same time hominids learned to walk and their pelvis became narrower.

Owen Lovejoy of Ohio State University has theorized that hominid males that got around on two legs got more sex than tree-bound rivals and produced more offspring because walking freed their hands and this gave them an advantage in finding food and carrying it back home. Being freed of food gathering responsibilities allowed females to devote more time to rearing children that grew up healthy and strong.

Chimpanzees and other apes have opposable big toes that allow easily grasp tree and climb. They also allow baby chimps to tightly grip their mothers with four limbs, which allows a the mothers to forage, escape danger and travel while keeping the baby close. Lacking opposable big toes is an advancement that has made walking easier and has been found in advanced hominids but it also has made life more difficult for mothers who had to hold their babies and rely on others to gather food for them.

Many of the aches and pains that humans experience with backs, knees and feet can be traced to bipedalism, walking and the pressure of gravity on an upright body. The spine, for example, was originally designed as a horizontal arch to carry weight along its length not a vertical column that carries weight above it. The upright design puts a lot of pressure on the lower regions of the spine, resulting in pressure and sometimes pain on the lower back. No other creatures experiences the kind of back pain that humans do. In the switch to bipedalism humans also gave up stability and speed and the ability to use the foot as a grabbing instrument.

One of the biggest consequences of bipedalism has been its impact on childbirth. A small hole in the pelvic bone is more advantageous for walking, running and supporting an upright body but is not so advantageous for giving birth to babies with relatively large brains and heads. The passage of the child through the bony canal of the human female pelvis is difficult and problematic. So narrow is the passage that the fetus most rotate as it moves through the canal and skull sometimes must compress by squeezing together, with the cranial bones that make up the skull overlapping. Getting the relatively broad shoulders of a human baby through the pelvic canal can also be difficult.

By contrast a chimpanzee infant exits its mother quickly and easily in a straight shot from the womb through the pelvic canal. The infant pops out face up so the mother and can pull it forward and immediately place the baby on her breast.

Apes and Humans

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apes and man

Michael D. Lemonick wrote in Time: “Ape “bodies are pretty much the same as ours, apart from some exaggerated proportions and extra body hair. Apes have dexterous hands much like ours but unlike those of any other creature. And, most striking of all, their faces are uncannily expressive, showing a range of emotions that are eerily familiar. [Source: Michael D. Lemonick, Time, October 1, 2006]

“It isn't just a superficial resemblance. Chimps, especially, not only look like us, they also share with us some human-like behaviors. They make and use tools and teach those skills to their offspring. They prey on other animals and occasionally murder each other. They have complex social hierarchies and some aspects of what anthropologists consider culture. They can't form words, but they can learn to communicate via sign language and symbols and to perform complex cognitive tasks. When it comes to DNA, a human is closer to a chimp than a mouse is to a rat.

“In 2005 geneticists announced that they had sequenced a rough draft of the chimpanzee genome, allowing the first side-by-side comparisons of human and chimpanzee DNA. Already, that research has led to important discoveries about the development of the human brain over the past few million years and possibly about our ancestors' mating behavior as well.

Cellular and Genetic Differences Between Apes and Humans

Michael D. Lemonick wrote in Time: “In the 1960s that details of our physical relationship to the apes started to be understood at the level of basic biochemistry. Wayne State University scientist Morris Goodman showed, for example, that injecting a chicken with a particular blood protein from a human, a gorilla or a chimp provoked a specific immune response, whereas proteins from orangutans and gibbons produced no response at all. And by 1975, the then new science of molecular genetics had led to a landmark paper by two University of California, Berkeley, scientists, Mary-Claire King and Allan Wilson, estimating that chimps and humans share between 98 percent and 99 percent of their genetic material. [Source: Michael D. Lemonick, Time, October 1, 2006]

“In 1998 glycobiologist Ajit Varki and colleagues at the University of California, San Diego, reported that humans have an altered form of a molecule called sialic acid on the surface of their cells. This variant is coded for by a single gene, which is damaged in humans. Since sialic acids act in part as a docking site for many pathogens, like malaria and influenza, this may explain why people are more susceptible to these diseases than, say, chimpanzees are.

“A few years later, a team led by Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, announced that the human version of a gene called FOXP2, which plays a role in our ability to develop speech and language, evolved within the past 200,000 years--after anatomically modern humans first appeared. By comparing the protein coded by the human FOXP2 gene with the same protein in various great apes and in mice, they discovered that the amino-acid sequence that makes up the human variant differs from that of the chimp in just two locations out of a total of 715--an extraordinarily small change that may nevertheless explain the emergence of all aspects of human speech. And indeed, humans with a defective FOXP2 gene have trouble articulating words and understanding grammar.

“Then, in 2004, a team led by Hansell Stedman of the University of Pennsylvania identified a tiny mutation in a gene on chromosome 7 that affects the production of myosin, the protein that enables muscle tissue to contract. The mutant gene prevents the expression of a myosin variant, known as MYH16, in the jaw muscles used in biting and chewing. Since the same mutation occurs in all of the modern human populations the researchers tested--but not in seven species of nonhuman primates, including chimps--the researchers suggest that lack of MYH16 made it possible for our ancestors to evolve smaller jaw muscles some 2 million years ago. That loss in muscle strength, they say, allowed the braincase and brain to grow larger. It's a controversial claim, one disputed by anthropologist C. Owen Lovejoy of Kent State University. "Brains don't expand because they were permitted to do so," he says. "They expand because they were selected"--because they conferred extra reproductive success on their owners, perhaps by allowing them to hunt more effectively than the competition.

“After a rough draft of the chimp genome was published in the journal Nature several learned important things. First, they learned that overall, the sequences of base pairs that make up both species' genomes differ by 1.23 percent--a ringing confirmation of the 1970s estimates--and that the most striking divergence between them occurs, intriguingly, in the Y chromosome, present only in males. And when they compared the two species' proteins--the large molecules that cells construct according to blueprints embedded in the genes--they found that 29 percent of the proteins were identical (most of the proteins that aren't the same differ, on average, by only two amino-acid substitutions). The genetic differences between chimps and humans, therefore, must be relatively subtle. And they can't all be due simply to a slightly different mix of genes


Genetic Switched Differences Between Apes and Humans

Michael D. Lemonick wrote in Time: “This shockingly small number made it clear to scientists that genes alone don't dictate the differences between species; the changes, they now know, also depend on molecular switches that tell genes when and where to turn on and off. "Take the genes involved in creating the hand, the penis and the vertebrae," says Lovejoy. "These share some of the same structural genes. The pelvis is another example. Humans have a radically different pelvis from that of apes. It's like having the blueprints for two different brick houses. The bricks are the same, but the results are very different." [Source: Michael D. Lemonick, Time, October 1, 2006]

“Those molecular switches lie in the noncoding regions of the genome--once known dismissively as junk DNA but lately rechristened the dark matter of the genome. Much of the genome's dark matter is, in fact, junk--the residue of evolutionary events long forgotten and no longer relevant. But a subset of the dark matter known as functional noncoding DNA, comprising some 3 percent to 4 percent of the genome and mostly embedded within and around the genes, is crucial. "Coding regions are much easier for us to study," says Carroll, whose new book, The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, delves deep into the issue. "But it may be the dark matter that governs a lot of what we actually see."

What causes changes in both the dark matter and the genes themselves as one species evolves into another is random mutation, in which individual base pairs--the "letters" of the genetic alphabet--are flipped around like a typographical error. These changes stem from errors that occur during sexual reproduction, as DNA is copied and recombined. Sometimes long strings of letters are duplicated, creating multiple copies in the offspring. Sometimes they're deleted altogether or even picked up, turned around and reinserted backward. A group led by geneticist Stephen Scherer of the Hospital for Sick Children in Toronto has identified 1,576 apparent inversions between the chimp and human genomes; more than half occurred sometime during human evolution.

“When an inversion, deletion or duplication occurs in an unused portion of the genome, nothing much changes--and indeed, the human, chimp and other genomes are full of such inert stretches of DNA. When it happens in a gene or in a functional noncoding stretch, by contrast, an inversion or a duplication is often harmful. But sometimes, purely by chance, the change gives the new organism some sort of advantage that enables it to produce more offspring, thus perpetuating the change in another generation.

gorilla, orangutan skulls

Genes and the Brains of Apes and Humans

Michael D. Lemonick wrote in Time: “A striking example of how gene duplication may have helped propel us away from our apelike origins appeared in Science in September 2006. A research team led by James Sikela of the University of Colorado at Denver and Health Sciences Center, in Aurora, Colo., looked at a gene that is believed to code for a piece of protein, called DUF1220, found in areas of the brain associated with higher cognitive function. The gene comes in multiple copies in a wide range of primates--but, the scientists found, humans carry the most copies. African great apes have substantially fewer copies, and the number found in more distant kin--orangutans and Old World monkeys--drops off even more. [Source: Michael D. Lemonick, Time, October 1, 2006]

Another discovery, first published online by Nature in August 2006, describes a gene that appears to play a role in human brain development. A team led by biostatistician Katherine Pollard, now at the University of California, Davis, and Sofie Salama, of U.C. Santa Cruz, used a sophisticated computer program to search the genomes of humans, chimps and other vertebrates for segments that have undergone changes at substantially accelerated rates. They eventually homed in on 49 discrete areas they dubbed human accelerated regions, or HARS.

“The region that changed most dramatically from chimps to humans, known as HAR1, turns out to be part of a gene that is active in fetal brain tissue only between the seventh and 19th weeks of gestation. Although the gene's precise function is unknown, that happens to be the period when a protein called reelin helps the human cerebral cortex develop its characteristic six-layer structure. What makes the team's research especially intriguing is that all but two of the HARs lie in those enigmatic functional noncoding regions of the genome, supporting the idea that much of the difference between species happens there.

Human Two-Olds Versus Orangutan and Chimpanzees on Intelligence and Behavior Tests

Christine Kenneally wrote in the Washington Post, In 2007 “a group of scientists in Leipzig, Germany, announced a tantalizing study that compared the learning abilities of human children with those of chimpanzees and orangutans. Three apes were presented with an array of tests that tapped their understanding of the physical world and how it works; for example, they had to use sticks to get out-of-reach objects, they had to follow the gaze of a person to find a reward, and they were asked to tell the difference between various amounts of an item. Remarkably, chimpanzees and humans were typically either the best or equally good. But when the researchers measured social rather than physical intelligence, the field changed completely: Humans were significantly better at understanding other minds. [Source:Christine Kenneally, Washington Post, April 13, 2008]

Michael Tomasello wrote in the New York Times: You might think that human beings at least enjoy the advantage of being more generally intelligent. To test this idea, my colleagues and I recently administered an array of cognitive tests — the equivalent of nonverbal I.Q. tests — to adult chimpanzees and orangutans (two of our closest primate relatives) and to 2-year-old human children. As it turned out, the children were not more skillful overall. They performed about the same as the apes on the tests that measured how well they understood the physical world of space, quantities and causality. The children performed better only on tests that measured social skills: social learning, communicating and reading the intentions of others. [Source: Michael Tomasello, New York Times, May 25, 2008; Michael Tomasello is co-director of the Max Planck Institute for Evolutionary Anthropology]

“When you look at apes and children in situations requiring them to put their heads together, a subtle but significant difference emerges. We have observed that children, but not chimpanzees, expect and even demand that others who have committed themselves to a joint activity stay involved and not shirk their duties. When children want to opt out of an activity, they recognize the existence of an obligation to help the group — they know that they must, in their own way, “take leave” to make amends. Humans structure their collaborative actions with joint goals and shared commitments.

“Another subtle but crucial difference can be seen in communication. The great apes — chimpanzees, bonobos, gorillas and orangutans — communicate almost exclusively for the purpose of getting others to do what they want. Human infants, in addition, gesture and talk in order to share information with others — they want to be helpful. They also share their emotions and attitudes freely — as when an infant points to a passing bird for its mother and squeals with glee. This unprompted sharing of information and attitudes can be seen as a forerunner of adult gossip, which ensures that members of a group can pool their knowledge and know who is or is not behaving cooperatively. The free sharing of information also creates the possibility of pedagogy — in which adults impart information by telling and showing, and children trust and use this information with confidence. Our nearest primate relatives do not teach and learn in this manner.

“Finally, human infants, but not chimpanzees, put their heads together in pretense. This seemingly useless play activity is in fact a first baby step toward the creation of distinctively human social institutions. In social institutions, participants typically endow someone or something with special powers and obligations; they create roles like president or teacher or wife. Presidents and teachers and wives operate with special powers and obligations because, and only because, we all believe and act as if they fill these roles and have these powers. Two young children pretending together that a stick is a horse have thus taken their first step on the road not just to Oz but also toward inhabiting human institutional reality.

“Human beings have evolved to coordinate complex activities, to gossip and to playact together. It is because they are adapted for such cultural activities — and not because of their cleverness as individuals — that human beings are able to do so many exceptionally complex and impressive things.

“Of course, humans beings are not cooperating angels; they also put their heads together to do all kinds of heinous deeds. But such deeds are not usually done to those inside “the group.” Recent evolutionary models have demonstrated what politicians have long known: the best way to get people to collaborate and to think like a group is to identify an enemy and charge that “they” threaten “us.” The remarkable human capacity for cooperation thus seems to have evolved mainly for interactions within the group. Such group-mindedness is a major cause of strife and suffering in the world today. The solution — more easily said than done — is to find new ways to define the group.

Great Apes Have Midlife Crises Too, Study Finds

Melissa Healy wrote in the Los Angeles Times, “Researchers have found that chimpanzees and orangutans experience midlife crises just as surely as do humans. That finding, published in the Proceedings of the National Academy of Sciences, could upend firmly held beliefs about the roots of human happiness and the forces that influence its odd trajectory across the life span. "This opens a whole new box in the effort to explain" the midlife dip in well-being, said senior author Andrew Oswald, a behavioral economist at the University of Warwick in England. "It makes one's head spin." [Source: Melissa Healy, Los Angeles Times, November 19, 2012]

“An international team of primatologists from Scotland, Japan and Arizona and devised an unprecedented census of well-being among 336 chimpanzees and 172 orangutans of all ages living in two research centers, one sanctuary and nine zoos across five countries. To gauge the animals' well-being, the researchers turned to the keepers that knew them best and asked them a series of questions that might stymie even the most devoted dog or cat owner. Designed to capture the mood, sense of effectiveness and pleasure-seeking drive of apes across the life span, the questions were based on established methods of measuring human well-being but modified for this population.

“Keepers were asked to rate the positive or negative mood of each subject and to gauge the degree of pleasure the animal derived from social situations. A third question was how successful each great ape was in achieving its goals — whether winning a mate, commanding the attention of a fellow member of its social group or gaining hold of an out-of-reach toy. Finally, the study authors asked keepers to consider how happy they would be if they had to live as their chimpanzees or orangutans for a week.

When the composite well-being score for each ape was plotted according to his or her age, the result was the same distinctive U-shaped curve seen universally in humans. Around the ages of 28 and 35 — roughly the midpoint of the chimpanzees' and orangutans' expected life spans — moods sagged, animals became less socially engaged and they were less likely to persist in attaining the things they desired.

"I certainly was shocked," Oswald said.While highly sociable, great apes lack the hallmarks of humanity most often associated with a drop in mood and well-being at midlife — the ability to evaluate one's status relative to expectations, an awareness of one's mortality, and social and family responsibilities so burdensome they might induce stress. And yet, noted Weiss, middle-aged chimps and orangutans experience more anxiety and less pleasure than their younger counterparts, and they're just not as successful at getting what they want. "You'd probably see it in their posture," he said: They are just not loving life.

Great Ape Midlife Crisis and What It Says About Humans

Melissa Healy wrote in the Los Angeles Times, “If our animal relatives share our propensity for sadness, withdrawal and frustration at life's midpoint, perhaps the midlife crisis is actually driven by biological factors — not the wearing responsibilities of jobs and family and the dawning recognition of our mortality. For men and women alike, social science researchers have located the winter of our discontent somewhere near the 50-year mark, wedged neatly between the vigor and drive of youth and the quest for meaning and happiness that marks the final decades of life. More than just a cultural cliche, the midlife crisis is the well-documented nadir of human well-being on the U-shaped curve of happiness that stretches between birth and death. As happiness researchers have fanned out around the globe, they have documented this midlife trough in at least 65 countries, suggesting that it is a universal feature of human existence. [Source: Melissa Healy, Los Angeles Times, November 19, 2012]

Until now, however, the social scientists that have dominated this burgeoning field of study have drawn on economic, psychological and sociological explanations. By midlife, youth's hot-blooded drive to mastery has driven off. Responsibilities abound. Decades of striving — to raise a family, to establish oneself in the community, to climb the professional ziggurat — have shown us the mountaintop and, with it, the limits of our reach and usefulness. A recognition of our mortality settles in.

In the years after midlife, the theory goes, humans shoulder fewer burdens for the care of others. Their time horizons are shorter, prompting them to focus on people and activities that give pleasure and meaning to their lives. They regret less. Oswald had a hunch that these explanations were overlooking the fundamental role of biology in influencing mood. So he reached out to Alexander Weiss, a primatologist and evolutionary psychologist at the University of Edinburgh in Scotland who studies well-being in great apes.

For social scientists who saw shifts in happiness in strictly human terms, the findings were a forceful reminder that people have not evolved as far as we may think beyond the great apes, said Stacey Wood, a neuropsychologist at Scripps College in Claremont who wasn't involved in the study. "It's a little humbling," said Wood, whose research focuses on the effects of age on happiness and emotional restraint. For animals that live in mutually dependent societies, the findings suggest that a midlife drop in sociability serves some positive evolutionary purpose, she said.

"It pushes more toward the possibility that this is biological," added Arthur Stone, a professor of psychiatry at Stony Brook University in New York who was not involved with the study. Whether it's hormones, brain structure, neurochemicals or some other factor that causes the middle-aged psyche to power down will require further research, Stone said. But from now on, he said, social explanations alone will not suffice. Oswald, who describes himself as "58 and very happily accelerating," said that for forlorn midlifers, the study is a happy reminder that while humans may be programmed to suffer a dip in pleasure, it gets better. "This suggests that it's completely normal, and that it's apparently out of your control," he said.

Image Sources: Wikimedia Commons

Text Sources: National Geographic, Natural History magazine, Smithsonian magazine, Wikipedia, New York Times, Washington Post, Los Angeles Times, Times of London, The Guardian, Top Secret Animal Attack Files website, The New Yorker, Time, Newsweek, Reuters, AP, AFP, The Economist, BBC, and various books and other publications.

Last updated April 2014

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