Human Evolution

The incredible story of our evolution from ape-like ancestors spans 6 million years or more, and features the acquirement of traits from bipedal walking, large brains, hairlessness, tool-making, hunting and harnessing fire, to the more recent development of language, art, culture and civilization.

Over the last century, many spectacular discoveries have shed light on the history of the human family. Somewhere between 12 and 19 different species of early humans are recognized and new specimens continue to be discovered. The tale of our ancestors is a tale of who we are, why we are and what it is that truly makes us human.

Human Eye

How could something as complex as the human eye develop naturally? More importantly, how could it develop in gradual stages while still being functional? The eye's evolution is actually quite simple, starting as primitive light-sensitive cells with gradual improvements over about 364,000 years.

Not only have scientists discovered how such a complex eye could have evolved from a light-sensitive cell, proving the process is not impossible, but every stage of the eye's development can be seen in nature!

Eye Evolution

Pigment Spot
(flat piece of light-sensitive cells)
The most primitive type of eye can be found on euglenids, small single-celled organisms found in the greenish scum in water. The euglenid has a bright red structure called an "eyespot" that allows the organism to move in response to light.

Function: A flat piece of light-sensitive cells on the skin helps an organism detect a light source.

Simple Pigment Cup
(flat piece of light-sensitive cells)
As a flat piece of light-sensitive cells deepen it forms a simple cup shape like the eye of a flatworm.

Function: A depression in the light-sensitive cells gives the ability to detect objects and the angle light is coming from.

Simple Optic Cup
(flat piece of light-sensitive cells)
As the depression increases it forms a simple optic cup like the eyes of abalone shellfish.

Function: The greater the cup shape of a depression the more precisely it can detect the angle of light. Edges shade the light so the angle can be based on what cells are shaded and what cells detect the light.

Pinhole Opening
(flat piece of light-sensitive cells)
As the opening becomes even smaller it forms a pinhole opening like the eye of a chambered nautilus.

Function: A smaller pinhole opening produces a sharper image and greatly increases the ability to detect the angle of a light source as less light-sensitive will be activated at a given moment.

Complex Lens Eye
(flat piece of light-sensitive cells)
As part of the transparent liquid that fills the eye became denser it formed a lens. Little by little the image became sharper. This type of eye can be found in marine snails and the octopus.

Function: A lens gives a sharper and clearer image.

Mammalian Eye
(flat piece of light-sensitive cells)
As the slight and gradual changes continue just a bit more the dense liquid forming the lens also takes shape as the iris and we now have an eye similar to that of land mammals.

Function: The new shape of the lenses gives an even sharper and clearer image still.

Walking Upright


	Continuing from knuckle-walking to walking upright gave us many evolutionary advantages when returning to the ground after climate changes decimated the forest, leaving wide belts of open terrain with no trees. Being up on two legs allowed us to see further and also freed up our hands to carry tools and perform other tasks. It also conserved energy. Metabolic and biomechanical comparisons indicate that humans walking on two legs consume only a quarter of the energy that chimpanzees use while knuckle-walking on all fours.

It is important to note that knuckle-walking, as chimps do, is actually the transition stage between walking on four legs to walking on just two. Knuckle-walking developed to adapting to life in trees. The continued change from that stage to walking on just two legs actually started while ancestors lived mostly and trees and then improved during increased time on the ground. Some of the changes required for more efficient bipedal walking (walking on 2 legs) were a skull placed on top of the spine, not in front, and thighbones angled inward to place support directly under the ribcage and over the feet. Also a broader pelvis angled to support internal organs with different muscle attachment points and s-shaped spin to move the humans balance point over their feet was needed. These changes however did not come without a price!

The Price of Walking Upright

The transition to upright walking demanded a significant range of adaptations of the skeleton and muscles. But it was not quite as dramatic a transformation as it might at first appear. Almost all primates can sit upright, many can stand upright, and a few can even walk upright (although not for long, nor very efficiently). In other words, there has been a pattern in primate evolution of an upright body position -- whether clinging vertically to a tree trunk, leaping like a lemur, or swinging through the branches like an ape. The transition to bipedalism in hominids built on this evolutionary pattern; it didn't require the direct transformation of a true quadruped like a horse into a committed biped. Never the less modern human anatomy has built on the strengths and the weaknesses of a body plan inherited from distant ancestors, giving us an efficient and graceful gait but also a range of painful problems from back injuries to fallen arches.

The Spine

Our spines are a heritage from distant ancestors who carried themselves horizontally, in water and on land. In those ancient creatures, as in modern quadrupeds, the spine functioned more like a flexible suspension bridge, supporting the body's organs -- a role to which it is structurally well suited. The human spine has been transformed into a weight bearing column, putting it under unprecedented stresses and dooming us to the likelihood of back injuries and pain.

Birth Canal

Perhaps the greatest downside of walking upright is the difficult passage through the birth canal of a female pelvis. Unlike monkeys and apes, whose infants are able to move around independently soon after they are born, humans give birth to very helpless young. This is because human babies develop very slowly in the womb and are born at a stage when they are still very underdeveloped compared to the ape species. The brain of human newborns is only one quarter of its full size, whereas those of chimpanzee infants are already half grown at birth. Most of the baby's development therefore takes place outside the womb, with the brain only reaching full size at around a year of age. Being born so early is what makes human infants so uniquely vulnerable and helpless.

The major reason why human babies are born so early is because of the size and structure of the mother's pelvis. For a species to walk efficiently on two legs, the pelvis must provide very stable support for the upper body and the internal organs. This has given rise to a bowl-shaped pelvis in modern humans that works very effectively as far as walking is concerned, but creates some major problems for giving birth. It restricts the size of the birth canal and even causes it to be twisted. The inlet to the birth canal is widest from side to side, so the baby enters it facing sideways. However the outlet of the birth canal is widest from top to bottom, so that halfway through birth, in order to squeeze through the outlet, the baby has to rotate so that it is facing downwards. Even after the twisting the opening is still too small hence why the baby must be born before it is fully developed. In some cases its cranial bones must squeeze together and overlap, compressing the skull a few millimeters so the infant can exit. Human birth is so complicated that before the age of modern medicine 1 out of every 5 births would result in the death of the mother. It's definitely not the type of system you would invent if you were designing it. But evolution is clearly a tinkerer, not an engineer.

Wrist Joints

After our ancestors left the trees and stood upright did their fingers lose their curvature and shorten relative to the thumb. This allowed fingertips to gain manipulative skills unique to our species, but the unique arrangement of the wrist joint leaves us vulnerable to such ailments as carpal tunnel syndrome.

Feet

What finally bears the full weight of our upright body are the two ridiculously tiny platforms we call our feet. The foot has a very narrow window for working correctly. In people with a reduced arch, fatigue fractures often develop. In those with a pronounced arch, the ligaments that support the arch sometimes become inflamed, causing plantar fasciitis and heel spurs. When the carrying angle of the leg forces the big toe out of alignment, bunions may form. This is more of a problem for women than men because of their wider hips. Also one of the greatest aspects of the human foot, compared with the chimp and other apes, is the relatively large size of its bones, particularly the heel bone. A 350-pound male gorilla has a smaller heel bone than a 100-pound human female. The gorilla heel bone is dense and solid with thick cortical bone while the human heel is puffed up and covered with only a paper-thin layer of cortical bone. The larger heel bone was developed to dissipate the stress delivered by normal bipedal walking but the redistribution in our bones means that humans have much more surface exposed of their skeletal tissues. This results in an accelerated rate of bone mineral loss as we age, which may eventually lead to osteoporosis and hip and vertebral fractures.

Our Other Changes

Evolution is about changes and our ancestors have gone through many changes before reaching this current stage in the evolution of modern man. These changes are the result of adaptation to the environment for sakes of survival. For example, climate actually dictates body shape. The colder the environment means the shorter and stockier a race generally is to retain heat. Because of these relatively recent adaptation is why we have much diversity visually amongst the human race. One of the most noticeable distinctions is skin color, ever wonder why there are so many different races of people with different shades of skin?

Human skin color ranges from almost black to nearly colorless which appears pinkish white due to the blood in the skin. Skin color is determined by the amount and type of pigment melanin in one's skin. People with ancestors from sunny regions have darker skin than those who were born in regions with less sunlight. Ultraviolet Light (UV) has both a positive and negative effect on us. On one side it can create mutations in skin cells causing skin cancer but we also must maintain a certain amount of UV to produce Vitamin D for strong bones. The first generation of modern man lived in the sun dominated land of Africa. As many of you know people living in Africa have very dark skin, this is because their bodies produce a larger amount of melanin to act as a light filter and protect them from the harsh UV light getting under their skin. As modern man travels out of Africa to other regions of the world further from the equator the UV light gets weaker. Since the UV is not as strong we must lower our protection against it so enough UV still gets in to allow for the production of Vitamin D. Our bodies lower its defense by lowering the amount of melanin inside our skin which also results in a lighter pigmentation of our skin.

Think of it like a pair of sunglasses. If the sun is really bright outside you want lenses with a dark enough tint to protect your eyes, but you don't want them to be too dark or else not enough light will get in for visibility. If you are indoors where it is not too bright you will want a pair of glasses that have a very light tint allowing more light through so you can still see. If you do not wear the right pair of sun glasses to adapt to your surroundings you may get blinded by the sun or it will be too dark to see where you are going. The amount of melanin in our skin is how we adapt to our surroundings. The more time you spend out in the sun the more melanin will build up causing a slight tan. As you know just moving to an area with high UV lights will not change our natural skin color nor noticeably affect our offspring. It actually takes about 20,000 years for a race with dark black skin to evolve to paler skin.

Our Journey

Homo sapiens are a VERY young species and came into existence very recently in Earth's history. If the entire history of the earth was laid out on a 12 month calendar with the formation of earth starting on January 1st and now being December 31st then humans would have first appeared about 15 minutes before midnight on new year's eve. All of recorded history would only stretch back to the last 16 seconds.

This map combines data from genetic relationships and archaeological and fossil data showing the first colonization of different regions by Homo sapiens. For a more detailed map click on the below map.

In just 7,000 the first modern humans left Africa and spread across the globe. This journey is traced by artifacts, fossils and an unbroken genetic line. In tracing that line we use markers within DNA that doesn't get mixed and shuffled at each generation, Mitochondrial DNA (passed on by only females) & Y chromosome (passed on by only males).

To learn more please vist:

The Genographic Project
by National Geographic

Journey of Mankind
bradshawfoundation.com/journey

DNA Ancestry Project
dnaancestryproject.com

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