Human beings, aka Homo sapiens, branched off from the evolutionary tree of our primate cousins some 200,000 years ago. However, fossil evidence suggests that our various hominin predecessors were extant even millions of years ago. And a multiplicity of fields are necessary to decipher our origins. Specialized techniques are used to excavate fossils, while technologies such as geomechanical dating, and databases in genetics, ecology, and paleoecology, and etiology are employed to piece together our story.

The start of human evolution is characterized by several changes, namely bipedalism, increased brain size, prolonged gestation, and decreased sexual dimorphism, or physical discrepancies between male and female. Although there is significant obscurity surrounding our exact ancestry, much progress has been made since Charles Darwin asserted his thesis on evolution in his seminal work, The Origin of Species, in 1859.

Glance through the pages of our lineage and learn our fascinating story – from the troop of ape-like animals roaming the African plains to the iPhone carrying, hashtag following humans we are today.

The Beginning of Man

Using the molecular clock, which approximates various events based on the rate at which a species accumulates genetic mutations, scientists estimate that our ancestors diverged from an apelike species during the Late Miocene Epoch. This was about 11.2 to 5.3 million years ago.

From Primate to Human

via: Getty

Among the species of primates that scientists propose were the first to kick this off were Kenyapithecus (hailing from Kenya) and Griphopithecus (originating from central Europe and Turkey).

The Miocene Epoch is marked by salient climate change, including the advent of seasons with colder winters north of the equator.

As weather patterns shifted, so did plant life and the diverse environments that grew out of this time period spurred numerous adaptations, such as bipedalism.

Bipedalism, or moving with two feet, is more than just a definition it’s a major factor to our success.

Whereas many bipedal animals can only hop or lumber around, humans actually move quite adeptly. This is accomplished through the shape of our pelvis, as its ilia will curve forward and situate the gluteal muscles on either side of the hip joint. This steadies the pelvis as each leg takes a step.

There are a variety of theories that attempt to explain why we evolved to be the only mammal who is solely bipedal.

Among the most compelling include increased ease of traveling long distances, thermoregulation, and providing protection and food to mates and offspring. Science has shown that human walking is about 25% more efficient than a similarly sized mammal, meaning that walking could have been selected over time to increase energy conservation. Walking upright also decreases the body surface area that is exposed to the sun, allowing early humans to regulate their body temperature on the plains of the Savannah. The third prominent hypothesis posits that as pair-bonding activities increased and their offspring became more demanding to raise, bipedal males could survey the land more easily for threats, as well as reach food growing in trees or bushes.

Our feet are also unique to other primates, as our heels are more defined and our arches are stable to provide the necessary support for walking.

As we stopped using our feet to grasp objects, our toes also became smaller. In addition, our big toes are responsible for absorbing some of the shocks of walking, making them sturdier and non-opposable.

Of course, our true defining feature isn’t our feet or our pelvises, but our intelligence.

Our brains became supersized beginning with homo habilis, around 2.3 million years ago and continued to expand until its current size, which is about three times that of a chimpanzee’s. However, it’s not the size that matters, it’s the function.

There are many different theories as to how we evolved better brains than many other creatures.

Though our brains only account for 2% of our body weight, it requires 20% of our energy, so one can imagine the cost of this evolutionary strategy. Furthermore, Neanderthals, who were the last remaining species of hominin before Homo sapiens supplanted them, actually had larger brains than their competitors! But I digress – the four major categorical theories on human cerebral evolution are as follows: ecological, cooperative ecological, interpersonal competition, and intergroup competition. The most compelling evidence supports an ecological model, suggesting that our response to our environment was more influential than our response to each other for shaping our brains. One possible explanation for this result may be that as a human or group of humans increase their skills, they’re continually pitted against the increasing skill of other humans, causing diminishing returns. However, environmental challenges are more stable, meaning that the energy allocated towards these pursuits are far more rewarding.

Our brains contain several categorical circuitries that can be generally defined by the complexity of their role.

The hindbrain is located near the spinal cord and controls our most basic functions, like breathing. The midbrain contains emotion processing units, such as the Limbic System, and the forebrain governs reasoning and language. The neocortex is the net of gray matter that covers the cerebrum and represents the ridges and grooves of our brain. Human intelligence is largely credited to the increased size of our forebrains as well as the volume of our neocortex. For example, about 36% of the human brain is dedicated to the forebrain and our neocortex is on average 3 mm thick, whereas 29% of a primate’s brain is composed of their forebrain and their neocortex is about 2.5 mm thick.

What Caused This Rapid Change in Brain Size?

Scientists believe that a particular protein called srGAP2, which is responsible for neuronal migration and differentiation, played a critical role in our encephalization. The gene that encodes this protein has been duplicated three times in the human genome within the past 3.4 million years and after its second duplication, the size of the hominin braincase began to increase rapidly.

Beginning around 2.6 million years ago, certain species of hominins in east Africa were creating and using simple tools to fulfill their needs.

For example, fossilized bones found in Ethiopia show evidence of being cut and pounded by stone tools. And since lower primates can be observed using tools themselves, scientists speculate that our unusual fine motor control and dexterity evolved in tandem to our use of increasingly intricate tools (likely due to our increasingly intricate brains!).

It doesn’t take having a human-sized brain to know that we are also unique because of our opposable thumbs.

But thoroughly understanding why our thumbs are special is a little more complex. In truth, we are not the only primates that have opposable thumbs, as Old World monkeys such as baboons, and all Great Apes (which include chimpanzees and gorillas) also have them. However, what’s missing in other primates is an ability to exert as much strength and precision when they use their thumbs. Humans have a particular muscle called the flexor pollicis longus, which stretches from the inside of our elbow and terminates near our wrist, that permits our thumbs to exercise great strength. Our thumbs are also more flexible than the average primate, as the pads of our fingers and thumbs are able to hyperextend. Scientists estimate that these features fully evolved with the Homo habilis species 1.8 million years ago.

Most of us probably don’t appreciate just how important tools are in our daily life.

Did you eat today? You probably used some kind of tool to help you, like a fork or a set of chopsticks. Did you write, brush your teeth, or comb your hair? You used a tool for those tasks, too! In fact, our expanding brain size is so closely correlated with the sophistication of tools that many scientists weigh it as the most compelling indicator of our evolution. Beginning about 2.5 million years ago during the early Paleolithic period, humans began using tools such as hammerstones, flake tools, and core choppers. During what’s known as the Acheulean period, tools continued to refine and bifaces such as axes and spearheads surfaced.

Here is exactly where humans dispersed to by the time they were creating tools.

Evidence suggests that Homo ergaster was the first hominin to leave Africa and migrate to southern Eurasia, nearly 1.75 million years ago. Shortly thereafter (at least by means of evolutionary time), Homo erectus made it to Southeast Asia, about 1.6 million years ago. This migration is supported by the Saharan Pump theory, which posits that a land bridge from the horn of Africa to the Levant permitted flora and fauna to settle new regions. Homo heidelbergensis first populated western Europe around 350,000 years ago. Interestingly Homo floresiensis, whose fossils date back 100,000 years ago and are distinct to the Flores island in Indonesia, has an uncertain migration pattern.

As compared with other species in our phylogenetic tree, humans are far less sexually divergent

For example, the average male gorilla is 1.5 times larger than their female counterparts, whereas modern human males are only about .15 times larger than females. Smaller canine teeth are also observed, and less pronounced brow ridges are typical (thank goodness, because our ancestors were admittedly ugly). Human females are also the only species of primate who are fertile year round and who have a hidden estrus, or a fertility cycle that is not visually apparent (see the Celebes Crested Macaque for an abrupt example of what I’m talking about). Scientists theorize this change occurred in parallel with our increasingly complex social patterns and the demand of our offspring for dual parenting.

Few factors are more credited to our survival and success than our ability to make fire.

Precisely when we first conquered the flame is a hotly contested fact, though. Prior to recent discoveries, the earliest evidence of a hearth dated back to 300,000 years ago in Israel’s Qesem Cave. Yet inside South Africa’s Wonderwerk Cave, artifacts of scorched bones and ashed plants push the date back to approximately 1 million years ago. This would credit Homo Ergaster or Australopithecus Robustus as the first hominin to create fire, not Homo sapiens and Neanderthals, as previously thought.

Why Fire is so Important?

Some answers may be obvious (it keeps us warm) but others may not be! Scientists also believe that fire helped our brains to grow bigger. Cooking food meant that our ancestors were able to eat certain fare that would have otherwise been poisonous. And because cooking makes food easier to digest, they could spend less energy on digestion and more on let’s say, higher-minded pursuits. Further, many scientists also believe that the heat from fire helped reduced our need for body hair, allowing earlier hominins to run longer distances without overheating.

How did our head shape transform from broad cheekbones and heavy brow ridges to the more spherical and refined silhouette it is today?

Researcher Daniel Lieberman believes that our bipedalism had something to do with it. For example, a modern human head has a relatively flat, small face that is positioned directly under the braincase, which is easier to balance while running or walking. Our jaws are also much more delicate and our teeth are smaller than our ancestors. Lieberman suspects that this is because our cooking practices reduced the necessity for strong jawbones and teeth.

Modern humans belong to the species Homo sapiens, meaning “wise man" in Latin.

This final branch in the human evolutionary tree likely evolved in Africa from Homo erectus some 200,000 years ago. Although a theory called the “multi-regional" model hypothesizes that Homo sapiens evolved in numerous locations on Earth, the simplest “Out of Africa" model is more supported.

The "Mitochondrial Eve."

Why is the Out of Africa theory more popular among scientists? First, Africa contains more genetic variation than the rest of the world’s population put together. This suggests that more hominins evolved in Africa than anywhere else, including our direct ancestors. Second, an interesting trail of genetic evidence traces back all humans to a single African female, living between 50,000 and 500,000 years ago. This genetic sleuthing examined mitochondrial DNA, which is only inherited from our mothers, and was able to trace it back to this aforementioned, “Mitochondrial Eve." Scientists believe that she was likely one of the few survivors after an evolutionary bottleneck which put pressure on populations.

Social Butterflies

Familiar with the saying, “shared hardship brings people together?" It sure seems so. Around 70,000 years ago a supervolcano in Sumatra named Mount Toba erupted. This may have led to a nuclear winter that blocked out the sun’s rays and caused heat and light to diminish. This event, as well as the Younger Dryas ice age circa 12,000 years ago, could have further compelled humans to cooperate and create tighter-knit social groups.

Neanderthals existed in Eurasia from 400,000 to about 40,000 years ago.

They are our closest relatives and exactly why they became extinct is unclear. Some evidence points to the possibility that Neanderthals were too specialized to survive outside of the Ice Age conditions they were adapted to. Neanderthals had stout bodies suited for conserving heat and broad, short noses perfect for warming and humidifying cold air. And although they were considered apex predators, it’s possible that many of the species they hunted died out, leaving Neanderthals vulnerable to starvation. In contrast, there’s evidence Homo sapiens had larger social networks that they could rely on for trade and support during difficult times. Homo sapiens also showed greater innovation in their tool industry, developing sewing needles for weaving and bows and arrows for hunting.

Neanderthal and Homo Sapien Relations

Although Neanderthals were eventually eclipsed by Homo sapiens, there’s evidence that the two species interbred – this long term transfer of genes from one species to another species’ gene pool is referred to "as introgression." Fossil evidence shows that Homo sapiens and Neanderthals occupied the same habitat in the Middle East 60,000 years ago, after the former left Africa (Neanderthals never lived in Africa). Today, populations in Europe and Asia show anywhere from 1 to 4 percent Neanderthal DNA in their genes. Interestingly, there is no Neanderthal mitochondrial DNA found in the human genome which suggests that breeding may have only been successful in female Homo sapien and male Neanderthal pairs.

Denisovan and Homo Sapien Relations

Denisovans are a new addition to the hominin family tree. In 2008, scientists discovered the 41,000-year-old bones of a female buried in Siberia’s Altai Mountains. After examining her DNA, scientists determined that she belonged to a species not previously known and named her after the cave she was found in. Modern humans from Asia and Melanesia contain around 5 percent Denisovan DNA and one of their genetic traits is even credited for Tibetans ability to live at such high altitudes. The mutation to the gene EPAS1 helps Tibetans to cope with up to 40% less oxygen than other humans, making their life at 13,000 feet in the air possible.

40,000 years ago, our ancestors generated another massive shift in our evolution.

The Upper Paleolithic period (Eurasia) or the Later Stone Age (Africa) marks the development of language, art, religion, and settlements. This age coincides with the appearance of Cro-Magnon peoples, whose remains were found in a French cave and are believed to be the first modern humans to populate Europe.

There’s evidence to suggest that human language began with Homo habilis, who developed some of the first tools

Although other animals certainly communicate, and mammals such as whales and dolphins display an impressive range of vocal frequencies that we are just beginning to appreciate, humans are nonetheless unique in our ability to form and use language. The jump in cognitive abilities this hominin experienced also likely contributed to language formation. As the connectivity between the various lobes of the brain increased so did the development of Wernicke’s and Broca’s areas, two vital brain regions for comprehending and producing speech, respectively. Later, as social networks and communication improved, writing developed. Historians believe that the first purpose of writing was to document business transactions.

When artistry emerged in the human evolutionary timeline is another subject for debate.

The first unequivocal evidence dates back to 43,000 years ago during the Upper Paleolithic period with cave art as well as figurines from the Aurignacian tradition. That said, artistic appreciation, as well as some more ambiguous evidence like this 500,000 year old shell displaying a zigzag etching, may prove that humans have been developing a collective artistic tradition since the days of Homo erectus.

Even Great Apes such as Chimpanzees display seemingly spiritual behavior such as mourning the dead and appreciating sunsets

However, scientists believe that our large prefrontal cortex is the key to our religious practices. This is due to our ability to understand causality and use abstract thought, evinced by our tool making industries, social networks, and use of language. Although evidence of religion depends on how it’s defined, the first generally accepted form is found in Paleolithic burials. Organized religion is thought to stem from the Neolithic revolution, beginning 11,000 years ago in the Middle East. At this time, human social structures transformed from nomadic to settled. This caused an explosion in population and technology, likely compelling groups to codify rules on morality.

After the Paleolithic era ended about 12,000 years ago, the Epipaleolithic period ushered in the first glimpses of modern human civilization.

The Natufian culture in the Levant provides the first evidence of sedentary society, cultivating various cereals that led to the first farming practices. Many historians even believe they established the first city in the world, Jericho! Despite the Natufians’ incredible contributions to human history, the Neolithic era more commonly symbolizes the advent of human civilization. Mesopotamia is a region between the Zagros Mountains and the Arabian Plateau that is widely credited as the site to fit this bill. In Greek, Mesopotamia means “between two rivers", an apt name given that the land is flanked by the Tigris and Euphrates. The people of Mesopotamia developed script a pantheon of gods, specific social customs and laws, agriculture and refined weaponry, and even the wheel!

By examining the evolutionary timeline of humans, we can appreciate the unique amalgam of forces we represent.

On one hand, we are animals, just as vulnerable to environmental pressures and instinctual desires as any other on Earth. Yet, on the other hand, our species is nearly omnipotent, revolutionizing technologies from simple stone tools to artificial intelligence. I’m both humbled and empowered by these facts, I hope you are too.