the story of us: how we became human and what it all means

a narrative for fourth and fifth grade students

(greatly in need of illustrative accompaniment)

Lauren W. Hasten

Professor of Anthropology

Las Positas College

Fall 2004

 

How Does Evolution Work?

The story of us begins with cells, because all living things are made up of them.  A cell is the smallest unit of life.  Cells take food in and send waste out; they grow, reproduce and die.  The  simplest creatures are made up of only one cell, but it takes millions of cells working together to make a person.  How do they know how to build that person? Deoxyribonucleic acid, or DNA, gives them instructions. 

DNA is a lot like an alphabet with only four letters: A, which stands for the base called adenine; T, for thymine; G, for guanine; and C, for cytosine.  It takes three letters, or bases, in a row to spell out the recipe for making a particular amino acid, and when amino acids combine, they form proteins.  All living things are made up of mostly proteins; they stack up to make things like skin and teeth and hair.  With only four letters, the DNA alphabet provides enough information to build your whole body!

DNA is stored in the nucleus -- the control center -- of each cell.  It doesn't just float around there, though; it's found in long strands called chromosomes.  Chromosomes always occur in pairs.  One member of each pair comes from your mother and the other comes from your father.  Each chromosome has enough DNA to tell your body thousands of things, and each pair has its own set of responsibilities.  There might be a spot on one chromosome pair that is in charge of telling your body what type of hair to grow.  The chromosome that came from your mother might be set for curly hair, while the chromosome that came from your father might instruct your body to produce straight hair.  Curly hair usually wins out, but not by a lot -- so you wind up with wavy head full of compromise. 

We can see chromosomes under a microscope.  They look a little bit like earthworms.  Fruit flies have 4 pairs of chromosomes, while certain flowers have hundreds.  Chimpanzees, though, have 24 pairs of chromosomes.  Many scientists think that two of their pairs combined a very long time ago to produce a creature with only 23 pairs -- us!  They have shown that chimpanzees and human beings share more than 99.4% of our DNA, while gorillas share only about 97% of their DNA with both of us.  That means that human beings are more closely related to chimpanzees than chimpanzees are to gorillas -- even though both are apes!  That's part of the reason that scientists think human beings and modern chimpanzees evolved from the same ancient ape ancestor. 

So how does evolution happen?  It has a lot to do with having babies.  If a chimpanzee lives in a place where food is hard to come by, only the ones who get enough to eat will survive.  The smart ones will be able to find enough food to live long, healthy lives.  They will mate many times and have many babies, all of whom stand an excellent chance of being as smart as their parents are.  The less intelligent individuals, however, will not find as much food.  They will likely live shorter lives and have fewer babies.  If the more intelligent chimpanzees consistently have more offspring than the less intelligent ones, over time, the group as a whole will get smarter and smarter.

A very, very long time ago -- some say as long as 10 million years ago -- a band of apes lived in the trees. Their bodies were built for that type of life, with curved fingers and toes just perfect for hanging onto branches.  When the apes were on the ground, they got around on all four limbs; just like modern day chimpanzees, they were quadrupedal.  Like chimpanzees, they could probably hobble around on two legs for a while when necessary, but it wasn't easy and it made them very tired. They didn't need to, though, since they spent most of their lives in the trees.  They were perfectly evolved to fit into their environment.

Now one knows why for sure, but eventually the apes came down out of the trees.  Maybe the weather changed and the trees disappeared, or maybe the apes simply liked the food on the ground better.  In any event, it seems to have worked out for them.  Hands, they discovered, were useful for carrying things like babies and food, so they began the habit of walking on two legs; they became bipedal.  This made them taller, which happened to be useful for seeing things like predators and prey.  It also made them hominids, which means they were no longer apes.  They were not quite human yet, but they had set out on their way. 

What's the difference between a hominid and a human being?  Humans have language, art, religion, music, science and all of the other things that make us unique creatures on this earth; hominids are simply bipedal cousins of the apes.  All humans are hominids but not all hominids are human.

Among the first hominids, those who were better at getting around on two legs had a terrific advantage over the others (seeing the lion before it saw them, for one thing), so they probably lived longer and had more babies. Some were born with straighter feet and toes, which made them especially good bipeds.  Over time, they outnumbered the others.  Some were born with straighter hands and fingers, which made them especially good at handling objects (like food) and making things (like tools); over time, they, too, outnumbered the others.  Eventually everyone had flatter feet and more flexible hands.  They were probably our great-great-great-great-great-great-great-great- (imagine the word "great" here 100,000 times) grandparents. 

 

How Do We Know How Old Fossils Are?

How do we know all of this?  Fossils tell us the story.  When a bone is buried in the earth, sometimes it is replaced, bit by tiny bit, by the minerals that are present in the soil.  Over time, the bone turns into solid rock.  It still looks like a bone, but it has become a fossil.  Any living thing can become a fossil, but very few things actually do.  It takes very special conditions to make it happen -- like the dryness of a desert or the ash of a volcano.  Luckily for scientists, a good many hominid fossils have been found. 

How do we know how old a fossil is?  There are two basic ways.  We can try to give a fossil a relative date, which will tell us only that it is older than or younger than something else, or we can try to give it an absolute date, which tells us the actual age of the fossil, give or take a few thousand years. 

Why is it that the deeper you dig a hole in the ground, the further back in time you go?  It's because the earth is always busy making piles of dirt.  A river rises in the winter and recedes in the summer; when it does, it leaves behind a deposit of sand, silt, stones or mud on the banks.  The following year, the same thing happens and another deposit is made, right on top of the last one.  On and on it goes until thousands of layers are piled upon each other and the oldest one is at the bottom.  The deeper down you dig, the older the layer you'll find.  That's a type of relative dating. 

Sometimes we can use the laboratory to find out almost exactly how old a fossil is.  Carbon is not alive, but it is found in every living thing.  The smallest amount of carbon (or anything, really) possible is called an atom.  An atom always has a certain number of protons and neutrons whizzing around its center; carbon is happy when it has six of each for a total of 12.  The funny thing is, carbon can happen with two extra neutrons, too -- but it isn't happy.  Those two extra neutrons keep shooting off in all directions from every atom of carbon 14 present, and there are millions.  Can you imagine all of those crazy neutrons flying past you and into you?  That's called radiation.  Carbon 14 is a radioactive isotope; an isotope is any atom that has the wrong number of neutrons.

Radiation causes an isotope to decay, which means that there will be less and less of it left over time.  The rate of decay, referred to as a half-life, can be measured very accurately; one half-life is how long it takes for any sample to decrease by half.  The half-life of carbon 14 is 5,730 years.  If you start off with 80 grams of carbon 14, 5,730 years later there will be only 40 grams, or half, left.  After another 5,730 years there will be only 20 grams left.  By the time 50,000 years or so have passed, there will not be enough carbon 14 left to measure.

All living things are constantly taking in and getting rid of both regular carbon and carbon 14.  When something dies, the carbon 14 stops moving through the body and begins to decay instead. Scientists can measure the amount of carbon 14 that remains in a fossil and compare that to a reasonable estimate of how much should have been there to begin with. Since the rate of decay, or half-life, is known, they can figure out how old the fossil is.  That's a type of absolute dating. 

Because of its short half-life, carbon 14 dating can only be used with confidence on fossils that are between 500 and about 70,000 years old.  How do we date fossils that are older?  Often, we don't.  We date the matrix, instead -- the material in which the fossil was found.  Many fossils have been found in places where there were active volcanoes.  Using radioactive isotopes, we can date the ash and rock from their eruptions.  Any fossil trapped underneath must have died at least that long ago.  

 

A Very Old Child

In 1924, an Australian teacher named Raymond Dart began working on the fossilized skull of small ape found in Taung, South Africa .  The skull had been discovered accidentally by workers at a limestone quarry.  Realizing it was very old indeed, they sent it to Dart for examination.  Since the skull was completely encased in hardened limestone, it took him more than four years of patient work, in his spare time, to gently scrape and pick it clean.  What emerged was almost certainly not the face of an ape.  Dart recognized immediately that the canine teeth of this creature were much smaller than any ape's would be; they were much more like those of a human child, about three years old.  Could this possibly be an ancient hominid, Dart wondered? 

What makes a hominid different from an ape?  What makes people different from chimpanzees?  Lots of things, including language, religion, and art --  but those things don't become fossils!  Usually all we find is teeth and bones.  What is the difference, then, between the skeleton of an ape and that of a hominid?  The skeleton of an ape is built specifically for getting around on all four limbs, while hominids are made for walking on two.  For one thing, an ape's arms are almost as long as her legs; hominid arms are shorter.  Try walking on all fours like an ape.  Apes are much better at it, but you are better at walking on two.

So how did Raymond Dart answer his own question?  If he had been holding a complete skull, he would have looked at the base, or bottom.  He would have seen that the hole where the spinal cord entered the skull was closer to the center of the skull than it was to the back of the head.  This hole is called the foramen magnum.  Any four-legged creature will have a foramen magnum that is closer to the back of the head than it is to the center; this allows the head to hang down comfortably while the animal is walking.  Human beings have a foramen magnum much more like the one that Dart would have observed in his fossil's skull -- much closer to center, which allows the head to balance better on top of an upright, or bipedal, body. 

Unfortunately, Dart's fossil skulll was not complete enough to reveal the position of the foramen magnum. Judging only then, by the very human teeth that the Taung child posessed, Dart decided that his find must have been a bipedal hominid. He named his find Australopithecus africanus, which simply means "southern ape of Africa ."  He knew it was no ape, though, because apes are not bipedal, nor do they have teeth like those of the Taung child.  In 1925, however, the world was not yet ready to accept as an ancestor a creature with a brain no bigger than a chimpanzee's; our brains are close to three times bigger.  Dart thought of his find as a sort of "missing link" between apes and humans.  He called it an ape because of its smaller brain, but scientists today think of it as a hominid.  Why?  Because other, more complete africanus fossils were discovered later on, and they were surely bipedal. 

 

The Australopithecus Family

Still, Australopithecus africanus was very different from us.  Neither her body nor her brain were much bigger than a chimpanzee's.  Her curved fingers and toes suggest that while she probably wasn't particularly great at running, she could scurry up a tree in a hurry when the need arose.  She lived about 2.5 million years ago, at a time when her kind were not the only hominids strolling around Africa on two legs.  In fact, apes had been evolving into hominids in Africa for a very long time by the time africanus showed up.  Ardipithecus ramidus was one of the earlier ones, preceding his Australopithecine cousins by hundreds of thousands years.  Like them, he had large teeth and a big toe that was probably set apart a bit from the rest of his foot, sort of like a chimpanzee's.  What made him a hominid?  The position of his foramen magnum shows that he was definitely getting around on only two legs. 

While Australopithecus africanus and Ardipithecus ramidus are very important finds, scientists have more enthusiasm for africanus' close cousin, Australopithecus afarensis.  Fossils, as you know, are very hard to come by.  Sometimes scientists have to content themselves with a few bits of bone and a couple of teeth.  Australopithecus afarensis, however, left us two incredible gifts: a beautiful set of footprints forever sealed in hardened volcanic ash, and a near-complete skeleton known as "Lucy."  From these finds, scientists can be certain that bipedal hominids were present in Africa from at least 4 million years ago. 

 

The First Truly "Human" Beings

It took about two million years for evolution to make something different out of the Australopithecines.  How did it happen?  Little by little, over a long period of time, small changes added up to produce bigger and bigger changes until finally something very new and different was born.  Hominids grew taller and straighter as they became better suited for walking and running.  Their big toes came up straighter, moving into line with all the rest.  Their teeth grew smaller too -- especially the canines -- as they got better at finding and preparing different kinds of foods.  Most importantly, their brains got bigger -- much bigger than one would expect just by looking at their bodies.  They became hominids who were similar enough to us to be called Homo, the first truly human beings. 

In 1963, Louis Leakey found a fossil hominid unlike any he had seen before.  With only a slightly bigger brain and body than an australopithecine, it wasn't so much the size of the creature that impressed him, but its deeds.  For nearby, Leakey's team of archaeologists had found some of the earliest stone tools ever discovered.  They were crude and simple to be sure, but they were far beyond anything an ape could produce.  Since no tools had ever been found near an australopithecine, Leakey concluded that his new hominid must have been the first toolmaker.  He christened him Homo habilis, which means "handy human."  In so naming, habilis became the first hominid in the fossil record to be thought of as a human being. 

Even so, habilis was still very different from us.  He lived in Africa between 1.5 and 2 million years ago, getting by on a mostly vegetarian diet. He ate meat whenever he got the chance though. Since he had no claws or weapons, he could not have been much of a hunter, so he was probably happy to scavenge some meat when he happened upon the leftovers of another animal's kill.  His own kills were likely restricted to small ground birds and rodents, as well as whatever fish were available. 

One clue to habilis' diet lies in his teeth.  Homo habilis no longer had the large, sharp canines of his ape and australopithecine ancestors; now he had teeth a lot more like ours. This means that his diet must have changed.  Since teeth are made for chewing, different kinds of teeth chew on different kinds of things.  Sharp canines are used for tearing tough foods like animal flesh, while bigger, flatter molars are used for grinding up things like seeds and grasses.   Animals like lions who eat only meat are called carnivores; carnivores always have very sharp, pointed teeth.  Animals like cows who eat only grasses are called herbivores; herbivores have mostly big, grinding teeth.  Apes and humans are omnivores, which means we can eat a wide variety of foods, both animal and vegetable.  We have both kinds of teeth in our mouths, but a lion has much larger canines and a cow has much bigger molars.

Apes and Australopithecines have canines that are bigger and sharper than ours are, but Homo habilis' canines are smaller and much more like ours.  Somehow, on the two million-year road to becoming Homo habilis, the australopithecine ancestors lost their larger canines.  Why?  Because eventually, they became more trouble than they were worth.    

How then, do teeth get smaller over time?  That's evolution at work.  Some folks have big teeth and some have smaller ones.  When the Australopithecines were alive, there must have been some advantage to having smaller teeth.  Perhaps big canines got in the way of eating the vegetable foods that were available at the time.  Those with smaller teeth would have been able to consume more food, which would have made them stronger and healthier.  They might have lived longer lives and had more children; those children likely were born with the same small teeth as their parents.  If people with small teeth nearly always managed to have more children than people with large teeth, eventually everyone would have small teeth. 

Homo habilis, handy though he was, remained like an australopithecine in a lot of ways.  While his tools helped him to better prepare food and perhaps even clothing, his arms and hands were still more suited to tree-climbing. He did not think to use his tools for building shelters or making hunting weapons, and he probably had not yet learned to speak. He was small and lean, more often prey than predator.  If you ran into him on the street, you would not think of him as human.  Some scientists think he should not have been called Homo at all. 

 

The First Fully Upright Human Goes on a Very Long Journey

Around the same time Homo habilis was busily making tools (from about 1.8 million years ago), another hominid showed up on the scene in Africa -- one whom scientists agree looked a lot more like us.  Homo erectus, or "upright man," was so named because she was the first hominid to stand and walk fully upright.  She was a lot taller and denser than habilis, and she had a much bigger brain -- big enough, in fact, to be the first hominid that everyone can agree was human.  Below the neck, erectus looked very much like we do.  Her feet and hands were much like ours, and her spine had the unique S-curve that keeps human beings so stable on two legs.   Erectus would have had a fair chance at beating you in a foot race! 

Her head, though, was a different story.  Homo erectus had inherited her teeth from habilis, perhaps, but not her diet.  For while her teeth were smaller than those of her ancestors, she had very large eyebrow ridges and a rather big bump on the back on her head.  Most of her kind even had a bony ridge running down the middle of the top of their heads.  All of these bony structures were there for the same reason: to anchor large muscles in place.  These muscles were connected to a heavy jaw to give erectus extra chewing power; she must have been eating some very tough foods indeed. 

While Homo erectus was the first fully upright hominid, her achievements did not end there.  During the one and one-half million years she walked the earth, her brain fairly doubled in size.  Her tools improved, as did her survival skills.  Perhaps because of this, she was the first hominid to leave the continent where she evolved; while none of her ancestors have yet been found outside Africa, erectus has turned up in both Europe and Asia. 

The question is not so much why erectus left Africa, but why her predecessors did not.  Hominids first evolved in Africa because that was the only place warm enough for a hairless, bipedal ape to survive without fire, shelter or clothing.  The earliest hominids, like apes, were at the mercy of nature.  If they were cold, they shivered.  Homo erectus was a creature of a different sort; when she was cold, quite likely she skinned an animal and made a coat.  While no clothing has been found, her tools were certainly up to the task. 

With a brain far bigger than Homo habilis, Homo erectus was a much better toolmaker.  For while habilis' toolkit contained choppers and scrapers with only one working edge, erectus made finely crafted points that were sharp on both sides and much better for cutting animal hides. She also made pointed awls that were good for punching holes through skins so they could be threaded together with sinew or fiber. 

Hunting large game was still difficult, though.  Since there were no long-distance weapons, the hunters had to get very close to their prey in order to kill it, risking their own lives in the process.  There is no evidence that men did any more hunting than women.  Since meat was so precious, it is more likely that when game appeared, every able-bodied person gave chase.  Often the best they could hope to do was run the beast off a cliff. 

When Homo erectus left Africa for the colder climate of Eurasia, she might have had more than clothes with her to keep her warm.  There is some evidence that she had learned to use fire.  Archaeologists have found layers of ash and charred bits of bone, but it is almost impossible to tell whether they were left by man-made or natural fires.  Still, if erectus did not possess the skills to start a fire, it is almost certain she knew what to do with one.  If lightning struck a tree and set a branch on fire, she was intelligent enough take that branch, set it in a safe place, and keep the fire going by adding more branches.  It kept her and her family warm, it cooked their meat, and it scared away predators. 

 

The Next Step Toward Us

Homo erectus lived from about 1.9 million to 27,000 years ago.  Like all other hominids before her, she became extinct.  That is to say that some of her kind evolved into something else, while those who didn't eventually died out.  During that long span of time, her way of life did not change very much.  Living mostly in caves, she doesn't seem to have ever gotten the hang of building herself a permanent shelter.  She left behind nothing but bones, stones and ashes. 

Yet some of her kind did evolve, and in Europe it's likely they became Neandertals.  Neandertals, long seen by many as brutes, were the first to do something most touchingly human: Somewhere around 100,000 years ago, they began burying their dead.  Before the Neandertals, hominids did not pay any particular attention to the bodies of the dead.  They were probably left for the carnivores to dine on.  Something in Neanderthal man, however, would not let him send his fellows to that fate. 

When a Neandertal died, his companions carefully placed him into the ground with his knees drawn up to his chest.  This is referred to as a flexed, or fetal position, and it is how babies lay inside their mothers before they are born; many sleep that way for a while afterward, too.  We do not know whether Neandertals used this position to remind us of birth or simply to save space.  Their bodies, however, were often buried along with grave goods such as tools and animal bones.  Among modern humans, such items are given to the dead for use in the next life.  While we cannot know for sure what the Neandertals were thinking, many people conclude that they were practicing one of the very first religions.

Ancient pollen grains have been found, showing that flowers were placed into the graves as well.  Were they there only to cover up the smell or for some more heartfelt purpose?  We may never know the answers, but we do know this:  Neandertals were thinkers.  All hominids before them thought only of life; the Neandertals thought also of death.  The knowledge of their mortality was the price they paid for being smarter. 

Neandertals lived in Europe and Asia from at least 230,000 to about 27,000 years ago.  While Homo erectus had reached heights over six feet, Neandertals were on average only five feet tall.  Still, it's a Neanderthal you'd want on your football team -- he'd look huge even without his gear on.  Neandertals were built to survive the cold, harsh climate of a Eurasian winter and their bodies were made for hard work.  They had massive bones and dense muscles, perfect for getting things done without the help of animals or machines. 

Even their brains were big -- bigger than ours, in fact.  Does that mean they were more intelligent than us?  Probably not.  Even today, people whose ancient ancestors lived in cold northern climates tend to be shorter, more stocky, and have slightly bigger brains.  Why?  Because it keeps them warmer.  What takes longer to freeze -- a glassful of water or a tray of ice cubes?  Try it for yourself and you'll see that the ice cubes freeze first.  That's because they have more surface area than the glass of water does; cold air hits them across much more space than it does the water in the glass.  The water in the tray gets colder in more places so it freezes much faster.  Neandertals were short and thick because they lived in very cold places; their dense bodies and bigger brains helped to keep them from freezing. 

Neandertals lived short, difficult lives; they were lucky if they lived to see forty years.  They had better tools than erectus but still lacked a long-distance weapon for hunting.  Nevertheless, they managed to work together to bring down huge mammoths, valuable for meat, clothing and shelter.  At one site in the Ukraine , Neandertals appear to have stood the curved rib bones of a mammoth up in an oval shape; it's likely they covered this frame with skins to make a home.  More often, though, they simply sought shelter at the mouth of a cave.  They may have been able to speak but if so, their language was probably limited.  Their way of life might have gone on much longer, were it not for the sudden appearance of a dangerous new predator: us.

 

What Happened to the Neandertals?

When Homo erectus left Africa, not everybody went with her.  Some erectus groups stayed behind in Africa, moving along on a different path of evolution.  Sometime around 200,000 years ago, these hominids began to look exactly like us; they became Homo sapiens sapiens, or "very wise human," the first of our kind.  While we are similar to erectus below the neck, the big difference is in our heads: our teeth and jaws are much smaller, and our brains are close to a third bigger!  Unlike the Neandertals, however, our bigger brains are not related to climate.  We evolved in a hot, dry place that favored hominids who were tall and lean.  That being the case, our brains should not be much bigger than erectus'.  But bigger they are, by a lot. 

Why, then, are our brains so big?  The answer is obvious:  Having a big brain is a tremendous advantage to someone who actually uses it to think!  Our bigger brains enabled us to build better tools and weapons, including the first sewing needles, spear throwers and arrows.  They gave us control over fire and the intelligence to learn how to grow plants, raise animals, and make medicines.  Most importantly, they gave us the power of language, which allows us to share what we learn with others.  Not to mention music, art and all the other beautiful things which give our lives joy and meaning.  With a brain such as this, we Homo sapiens sapiens are blessed with many abilities that allow us to live in our world successfully, no matter how much it changes. 

Blessings, though, often come with difficult gifts attached.  The gift of the Neandertal's big brain was the knowledge of his own death.  Our gift, unfortunately, is the power to cause death.  All hominids before us either changed with their environment or became extinct; we change the environment and cause extinction. Our big brains cause big problems, so they are certainly big enough to fix them.  With the gift comes the responsibility to do so. 

No one knows what happened to the Neandertals, but they seem to have disappeared shortly after we arrived on the scene.  There is no doubt that we were much smarter, since our tools and weapons were far more advanced.  We also had the advantage of being able to communicate better, since our language was much more developed.  Perhaps we were simply much better at surviving than the Neanderthals were.  Perhaps we were superior hunters who lived longer, healthier lives.  Or, perhaps we killed them.  History shows that we human beings are capable of killing strangers who frighten us or stand in our way -- especially if they look very different.  There is, however, absolutely no evidence to support this conclusion.

 

The Gift of the First Homo sapiens sapiens

We will probably never know what happened when the first Homo sapiens sapiens faced Neandertals eye to eye, but we do know this:  Modern humans are capable of great compassion and tolerance and always have been.  While we can indeed be destructive, we are creative beings first and foremost.  The biggest difference between Homo sapiens sapiens and every other hominid before us is the fact that we make things, and not just because we need them. 

Art is perhaps the ultimate human achievement.  While tools allow us to extend and improve our lives, art exists for pure enjoyment. Even chimpanzees have tools (of a primitive form, to be sure), but only Homo sapiens sapiens have art.  No hominid before us has ever taken the time to create an object whose sole purpose was to exist.  We, on the other hand, have been placing our inky handprints on walls since our birth on this planet.  "I was here," a thousand palm prints scream, "one hundred thousand years ago.  Don't forget about me!" 

Ancient sapiens sapiens have left us many beautiful works of art, from elaborate animal cave paintings to small sculptures of large women.  Some believe the paintings were part of a ritual done to guarantee success in the hunt; they might easily have been done for pure enjoyment, just as people paint today.  The sculptures, some feel, represent a great female goddess who was known across a large part of Europe.  In an age when obtaining food was still a difficult task, they might just as easily have been everybody's idea of the perfect woman: well fed.  In any event, one thing is clear.  In becoming artists, Homo sapiens sapiens had moved beyond merely surviving.  We had finally become the unique creatures that we are, capable of making anything we need, communicating anything we are thinking, and contemplating the very nature of our existence. 

 

Glossary

absolute date - A range of dates which tells us almost exactly how old a fossil is.  It is often determined through the use of radioactive isotopes. 

amino acid - A natural substance that stacks up to build proteins.

archaeologist - A scientist who studies the ancient remains of people and things in order to figure out what life was like before the invention of writing.

Ardipithecus ramidus - One of the oldest hominids discovered so far, dating back at least 4.6 million years. 

atom - The smallest amount of any substance possible.  An atom has protons and neutrons at its center and an outer shell of electrons. 

Australopithecine - An early hominid with a small body and brain who lived in Africa between 4.5 and 1 million years ago.

Australopithecus afarensis - An early Australopithecine, present in Africa between 3 and 4 million years ago. 

Australopithecus africanus - A middle period Australopithecine, present in Africa between 2 and 3.5 million years ago.

base - The basic building block of DNA, of which there are four: A, or adenine; T, or thymine; C, or cytosine; and G, or guanine.

biped - An animal who walks on two legs.

canine teeth - Counting back from the space between your two front teeth, the third teeth back on both sides, top and bottom.  They are the most pointed teeth in your mouth because they are used for tearing through foods.  

carnivore - An animal who eats only meat.

cell - The smallest unit of life.  Cells perform all of the activities necessary for life including growth and reproduction. 

chromosome - Found in the nucleus of a cell, a chromosome is a long strand of DNA bound together by proteins. 

decay - To lose matter over time.  Unstable isotopes lose matter due to radiation.

DNA - The substance that provides instructions for building living things.

evolution - Changes built up over time which eventually create new and different living things.

fetal position - Curled up on your side with your knees drawn to your chest.  This is the position in which babies grow inside their mothers. 

foramen magnum - The hole in the base of the skull through which the spinal cord enters.  It is much closer to the center of the skull in bipeds than it is in quadrupeds. 

fossil - The hardened remains of a living thing. 

grave goods - Items laid into a grave along with the body.  Mourners sometimes leave them for the dead to use in their next life.

half-life - The rate of decay for a radioactive isotope; the amount of time it takes for a sample of a radioactive substance to decrease by half. 

herbivore - An animal who eats only plants.

hominid - An ape-like primate who walks on two legs.  Monkeys, apes and humans are all primates, but only humans are hominids.

Homo - The specific type of hominid that we are: human beings.  Members of the genus Homo show the capacity for complex thought and creative behavior. 

Homo erectus - "Upright human;" the first fully upright hominid with a fairly big brain.  Found in Africa from at least 1.9 million years ago, erectus is believed to be the first hominid to have moved into Europe and Asia . 

Homo habilis - "Handy human;" the first hominid named, by Louis Leakey, to the genus Homo.  Leakey believed him to be the first hominid to make stone tools.  Habilis, however, did not have a particularly big brain. 

Homo sapiens sapiens - "Very wise human;" the only surviving species of hominid -- us.  Big-brained, tall and light-boned, we are the first hominids to control our own environments. 

human - A bipedal hominid with a large brain and the capacity for complex thought and creative behavior. 

isotope - An atom with a the wrong number of neutrons. 

matrix - The material in which is fossil is found, usually soil, sand or rock. 

Neandertal - An ancient hominid of the genus Homo, found only in Europe and Asia between 230,000 and 27,000 years ago.

nucleus - The control center of the cell, where chromosomes and DNA are found.

neutron - One of two types of particles at the center of an atom; the other is the proton.

omnivore - An animal who eats a variety of foods, both animal and vegetable.

protein - Made up of amino acids, protein is a basic building block for growing living bodies.

proton - One of two types of particles at the center of an atom; the other is the neutron.

quadruped - An animal who uses all four limbs to support his body while walking.

radiation - The loss of an isotope's extra neutrons.

relative date - The age of a fossil, relative to other things.  A relative date tells us only that a particular fossil is older than or younger than something else, but not by how much.