Much of the body we have today took shape millions of years before the first primate emerged. The basic machinery of life that we depend on evolved even earlier. Evolution happens when some random genetic change gives an individual some advantage in the competition of food and survival. Follow the timeline to see how evolution gradually assembled and tinkered with all the bits and pieces that make up modern human beings.
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Life Emerges (3.5 billion years ago)
How the first living cells emerged is still a matter for speculation, but 3.5 billion years ago, the first single-celled organisms exist. They're simple capsules of genetic material that can make copies of themselves -- and change over time. That's because instead of always making a perfect "clone," the copying system sometimes makes a mistake. And when these mistakes turn out to be useful, they give the new cell a novel ability that works to its advantage. Because these new cells then thrive and reproduce, the useful trait becomes established in the population. The ability to use the energy of sunlight to convert carbon dioxide into sugars evolved in this way early on, and when oxygen started being produced as a waste product, the Earth's atmosphere began to change. Over time, single cells evolved that could use oxygen, sense light and respond to chemical changes in the environment.
Inventing Sex (1.5 billion years ago)
The evolution of sexual reproduction was a huge leap forward. When a cell simply copies itself, it takes a mistake to introduce a genetic variation that might -- or might not -- be useful. But when an organism has sex to reproduce, two different sets of genetic material get randomly combined every time. This creates lots of opportunities for new characteristics to evolve.
The first organisms to have sex were single cells similar to modern yeast. These ancestors of yeast appeared about 1.5 billion years ago and show how much evolution has taken place since life first appeared. Each cell is now much more complex and organized. It has a nucleus containing DNA, and several different bits of cellular machinery, each of which plays a specialized role in keeping the cell alive. During this same time period, other single cells have started to join forces with each other to create the first multicellular creatures.
Our ancient yeast-like ancestors had already solved many of the basic challenges of being alive. One-third of the genes found in yeast have direct equivalents in us. Our bodies use sugars like yeast do; make hormones like yeast do; and have sex ... well, not quite like yeast do. The first basic building blocks of all animals, including humans, are now in place.
The Beginning Of A Body (530, 550 million years ago)
As colonies of cells evolve into multicelled creatures, the first bodies take shape. These early creatures are sac-like, but the cells they're made up from have organized themselves according to a simple body plan that all later animals -- including us -- have inherited. Their bodies have a front and a back, a top end with a mouth, and left and right symmetry. They contain nerve and muscle cells, but don't yet have specialized digestive, respiratory or circulatory organs, and no brain and spinal cord. But in evolutionary terms, they have many advantages over their simpler ancestors. They have some ability to move around and use other organisms as food. They are paving the way for the first predators.
Building A Backbone (530 million years)
This worm-like, underwater creature called Pikaea has acquired several more pieces of the evolutionary "lego set" that eventually built the first human. The most important of these is a stiff "backbone," and primitive brain and nervous system. These gave their owners much more control over their movements, and they could burrow as well as swim. They didn't have jaws, so they couldn't bite, but they did have a mouth, an anus and a rudimentary digestive system.
A Basic Body Plan (500-480 million years ago)
This fish may not look much like a human being, but it has most of our key body parts, systems and organs: It has a skull, jaw, spine and fins. The evolution of a bony skull and movable jaws was a huge step on the road to becoming both a modern fish and, eventually, a human. The ability to bite rather than just sucking and filtering water greatly increased the range of food it could consume, and created the first active predators. The appearance of paired fins -- which eventually became our arms and legs -- represented a huge advance in the physics of swimming. These fish also have gills for extracting dissolved oxygen from the water, and a circulatory system. Gills, and the structures that support them, eventually became key parts of our head, including our voice box and ears.
Preparing For Land (375 million years old)
Tiktaalik has begun to adapt to an entirely new environment: land. This ancestor is still a fish, but he has lungs as well as gills, so he could breathe in both air and water. The bones in his fins have evolved so they are sturdy enough and shaped to support and enable him to move around on land. This 375-million-year-old fish has all the major parts that will someday make a human being. From here on, evolution is tinkering with a basic construction kit.
Using Legs (375 million years old -- or younger)
Moving around on land rather than cruising through the water was a big challenge. Unlike water, land is much more varied -- there's mud, rocks, pebbles and plants to move over and around. The first tetrapods, or four-footed creatures, evolved to handle the new terrain: They all had four limbs with five digits at the end of each. Hearing in air is also very different from hearing underwater, so gill bones were repurposed into what became ear bones. The evolution of the first tetrapods set the stage for the evolution of all amphibians, birds and animals, each of which was adapted to a particular environment.
Warming Up (210 million years ago)
The evolution of mammals brought with it a whole slew of advantages, which opened up new environments and opportunities. Cold-blooded animals like reptiles have to rely on the sun to raise their body temperature enough to give them the energy to hunt for food. This limits the range of environments they can live in and the times of day, and seasons, during which they can be active. Mammals, on the other hand, can produce their own heat, which means they can function in a much greater range of environments.
Giving birth to live, dependent young was also a game-changer. Whereas reptiles lay eggs that basically hatch and then fend for themselves, mammals give birth to a more fully developed baby, and the mother produces milk to feed it. The longer a baby stays with its parents, the more time it has to develop -- and learn how to use -- a larger brain.
Agile Acrobats (Over 60 million years ago)
The major evolutionary changes taking shape in the first primates foreshadow what is to come. The brains of some of these squirrelly creatures may have been growing larger. Their hands and feet could curl around branches and grasp objects. This meant they could climb trees, as well as walk and run on all fours -- a big advantage. They could pick fruits and other foods that had previously been out of reach and evade predators. Fossil evidence suggests they were beginning to hold their heads and bodies upright when hopping and sitting. And their eyes were moving from the side of the head to become forward-facing, so it's likely that we were beginning to develop stereoscopic vision. This brought with it an increase in depth perception, improved hand-eye coordination, and the ability to judge distance. Ultimately, having eyes in the front of our faces created the opportunity for facial expressions and signaling our emotions.
Greater Versatility (20 -18 million)
Apes and monkeys are now following separate evolutionary paths. Our ape ancestors lost their tails and were getting bigger brains. It's also taking longer for their young to mature. Their bodies were developing flexible hip, ankle, shoulder and wrist joints. And they had the ability to "lock in" their elbows while the shoulder rotates. This allowed them to move freely through trees by hanging from branches above their heads. Later apes also developed broader chests and a stable lower back. All these changes meant that our ape ancestors already had much of the basic capacity in place for upright walking. Ultimately the flexibility in hand and shoulder joints allowed humans to throw spears and develop the physical skills required to craft tools. All modern apes, including humans, evolved from this early ancestor.
Upright Walking, Bigger Brains (5 million)
While humans are great apes, the great apes are not humans. Humans and chimpanzees -- our closest ape relatives -- started to diverge from a common ancestor between 8 million and 6 million years ago. This was followed by another 6 million years of human evolution. Fossils show that evolution experimented with a variety of early human "prototypes" before hitting on the version that led directly to us. These early human experiments all show similar trends, though: the transition to upright walking, bigger brains and more dexterous hands. They were clever toolmakers and thrived on a diet rich in nutrients. And at some point, they acquired the ability to think abstractly and use language.
Modern Humans (200,000 years ago)
Homo sapiens is the sole survivor of the early human family tree, which had many other branches that died out. Our ability to think abstractly and use spoken and written language has given us the ability to create and pass on new technologies and cultures very quickly. This means that in the past 200,000 years, we have adapted to, and taken control of, our environment at an unprecedented rate. Cultural evolution has outstripped physical evolution. We now live in a world of global communication and innovation. But we have changed the natural environment in multiple ways, and our population is growing at an unsustainable rate. The question is whether we will continue to evolve and survive.