Biology For Dummies

Part III It’s a Small, Interconnected World

Chapter 12

In This Chapter

Checking out your ancestors’ ideas of how organisms got here

Diving into Darwin’s controversial theories of biological evolution and natural selection

Examining the proof of biological evolution

Looking at both sides of the creationism-versus-evolution debate

If you’ve ever been to a museum, then you’ve probably seen fossilized bones or tools from ancient ancestors. These objects are evidence of how humans have changed and expanded their knowledge over the millenia. In other words, they provide perspective on how the human species has evolved. But what was the starting point of evolution, and from what did the earliest humans evolve?

This chapter tells you about the beliefs people had regarding evolution; how Charles Darwin came up with his theory of biological evolution; and what the current thoughts are on the origin of species, how humans have evolved, and how life on Earth began. Prepare to be amazed by the proof researchers have found for biological evolution and find out some pretty fascinating facts about how things used to be and how they’ve changed.

What People Used to Believe

From the time when ancient Greece was the world’s cultural hotspot until the early 1800s, philosophers, scientists, and the general public believed that plants and animals were specially created at one time and that new species hadn’t been introduced since then. (You could call this way of thinking fundamentalism.) In this view, every living thing was created in its ideal form by the hand of God for a special purpose. Aristotle classified all living things into a “great chain of being” from simple to complex, placing human beings at the top of the chain, just under the angels and very close to God.

People also thought the Earth and the universe it occupied were unchanging, or static, throughout time. They believed that God created the Earth, the stars, and the other planets all at once and that nothing had changed since the dawn of creation. These ideas held through much of human history, extending virtually unquestioned through the Middle Ages, when people even accepted that their place in society was unchanging and predestined by their birth.

Beginning in the 15th century and continuing into the 18th century, explorers, scientists, and naturalists made new discoveries that challenged the old ideas of a static universe.

Various explorers fell upon the New World (the Western Hemisphere of the Earth). The New World revealed many different species of living things, including new races of people, that were previously unknown. The New World and the people that lived there weren’t mentioned in the Bible, causing Europeans to debate whether the New World was created at the same time as the Old World and whether the people who lived there were descendants of Adam. These puzzles raised questions about a literal interpretation of the creation story in the Bible’s book of Genesis.

William Smith, a British surveyor, classified the types of ground material in Britain in preparation for the excavation of a canal system across the island. Smith discovered that the ground consisted of layers of different material and that different types of fossils could be found in each layer. He also found that the deeper he went into the layers, the more different the fossils appeared from the plants and animals that lived in Britain at the time. Smith used the fossils as a practical means of identifying the different layers of sediments, but he also made estimates of the age of the layers based on rates of erosion and the uplift of mountains.

Georges Cuvier, a French anatomist, demonstrated that fossil bones found in Europe, such as those of wooly mammoths, could be recognized as very similar to existing species, such as elephants, but were clearly not from anything currently living.

James Hutton, a Scottish geologist, proposed that the Earth was very ancient and that its surface was constantly changing due to erosion, the depositing of sediment, the uplift of mountains, and flooding. His idea, called uniformitarianism, was that the processes he observed on the Earth in the 1700s were the same processes that had occurred on the Earth since its creation.

How Charles Darwin Challenged Age-Old Beliefs about Life on Earth

Charles Darwin was a gentleman from the English countryside who set out on a seafaring journey on the HMS Beagle in 1831 as the ship’s naturalist. He was no less religious than others of his day, but he had a very active, curious mind and was acquainted with much of the scientific thinking of the time. That inquisitive mind combined with the scientific knowledge he’d gained over his short 22 years led Darwin to notice something rather unique about the finch population on the Galapagos Islands. These observations led to the creation of two of the most important biology-related theories of all time: biological evolution and natural selection. We fill you in on these theories and Darwin’s inspiration for them in the following sections.

Owing it all to the birds

While traveling on the HMS Beagle, Darwin visited the Galapagos Islands, which lie nearly 600 miles off the western coast of South America. He was amazed to find a variety of species that were similar to those in South America yet different in ways that seemed to make them exactly suited to the unique environment of the isolated islands.

Characteristics of organisms that make them suited to their environment are called adaptations.

Darwin chose to focus his attention on the Galapagos Islands’ finches (a type of bird). Each island had its own unique species of finch that was distinct from the other species and from the finches on the mainland. In South America, finches ate only seeds. On the islands, some finches ate seeds, others ate insects, and some even ate cactuses. The beak of each type of finch seemed exactly suited to its food source.

Darwin thought that all the finches had a common ancestor from mainland South America that either flew or floated to the newly formed islands, perhaps during occasional storms. The islands are far enough apart from each other that finches can’t really travel between them, so the different populations are geographically isolated from each other. Geographic isolation means they also can’t mate with each other and combine their genes (sequences in DNA that control the traits of living things; see Chapter 8).

Darwin proposed that each type of island had unique conditions and that these unique conditions favored certain traits over others. Birds whose traits made them more successful at obtaining food were more likely to survive and reproduce, passing their genes and traits on to their offspring. Over time, the characteristics of the island birds shifted away from those of their mainland ancestor toward characteristics that better suited their new home. Eventually, the island birds became so different from their ancestors, and from each other, that they were unique species.

What Darwin observed in the finch population in the Galapagos Islands is a type of biological evolution referred to as adaptive radiation. Adaptive radiation happens when members of one species get into environmental niches and have very little competition for resources at the outset. The lack of competition allows the species to become rooted in a new environment and increase its population. As the population increases, competition for resources begins, and the original species breaks off into several new species that adapt to different environmental conditions.

Darwin’s theory of biological evolution

Biological evolution refers to the change of living things over time (it’s not the same as evolution, which simply means change). Darwin introduced the world to this concept in his 1854 work, On the Origin of Species. In this book, Darwin proposed that living things descend from their ancestors but that they can change over time. In other words, Darwin believed in descent with modification.

As changes occur in living things, species that don’t adapt to changing environmental conditions may become extinct, or disappear. Species that accumulate enough changes may become so different from related organisms that they become a new species because they can no longer successfully mate with related populations; this process is referred to as speciation.

If you’re ever interested in reading Darwin’s On the Origin of Species (or any of his other works), check out darwin-online.org.uk.

The idea of natural selection

Darwin concluded that biological evolution occurred as a result of natural selection, which is the theory that in any given generation, some individuals are more likely to survive and reproduce than others. When a particular trait improves the survivability of an organism, the environment is said to favor that trait or naturally select for it. Natural selection therefore acts against unfavorable traits.

The theory of natural selection is often referred to as “survival of the fittest.” In biological terms, fitness doesn’t have anything to do with your BMI or how often you work out. Biological fitness is basically your ability to produce offspring. So, survival of the fittest really refers to the passing on of those traits that enable individuals to survive and successfully reproduce.

In the next sections, we help you understand the difference between natural and artificial selection, why natural selection can occur in the first place, and what the different types of natural selection are.

Comparing natural selection with artificial selection

Darwin compared his theory of natural selection with the artificial selection that results from selective breeding in agriculture.

Artificial selection occurs when people choose plants or animals and breed them for certain desired characteristics. Farmers in Darwin’s day bred the cows that gave the most milk, the chickens that laid the most eggs, and the pigs that got the biggest, creating many different breeds of each species. People’s preferences have dramatically shaped the breeds of domestic animals and plants in a relatively short amount of time.

Natural selection occurs when environmental factors “choose” which plants or animals will survive and reproduce. If a visual predator, such as an eagle, is cruising for its lunch, the individuals that it can see most easily are likely to be eaten. If the eagle’s prey is mice, which can be white or dark colored (see Figure 12-1a), and the mice live in the forest against dark-colored soil, then the eagle is going to be able to see the white mice more easily. Over time, if the eagles in the area keep eating more white mice than dark mice (see Figure 12-1b), then more dark mice are going to reproduce. Dark mice have genes that specify dark-colored fur, so their offspring will also have dark fur. If the eagle continues to prey upon mice in the area, the population of mice in the forest will gradually begin to have more dark-colored individuals than white individuals (see Figure 12-1c).

In this example, the eagle is the selection pressure — an environmental factor that causes some organisms to survive (the dark-colored mice) and others not to survive (the white-colored mice). A selection pressure gets its name from putting “pressure” or stress on the individuals of the population.

Reviewing the conditions under which natural selection occurs

In order for natural selection to occur in a population, several conditions must be met:

Individuals in the population must produce more offspring than can survive. Human beings are somewhat unique among living things in that we can make conscious choices about how many offspring we have. Most other organisms, however, produce as many offspring as they can.

Those individuals must have different characteristics. During Darwin’s time, no one knew where these differences came from. Now scientists know that differences in organisms arise due to mutations in DNA combined with the mixing of genetic information during sexual reproduction (for more information on genetic variation due to sexual reproduction, see Chapter 6).

Offspring must inherit some characteristics from their parents. During Darwin’s time, the laws of inheritance were just beginning to be figured out, so Darwin didn’t know exactly how parents passed on their traits. Modern scientists know that traits are inherited when parents pass genes on to their offspring (head to Chapter 7 for more on inheritance and Chapter 8 for more on genes).

Organisms with the best-suited characteristics for their environment are more likely to survive and reproduce. This is the heart of natural selection. If there’s competition for survival and not all the organisms are the same, then the ones with the advantageous traits are more likely to survive. If these traits can be inherited, then the next generation will show more of these advantageous traits.

If these four conditions are met, then the new generation of individuals will be different from the original generation in the frequency and distribution of traits, which is pretty much the definition of biological evolution.

Checking out the four types of natural selection

Natural selection can cause several different types of changes in a population. How the population changes depends upon the particular selection pressure the population is under and which traits are favored in that circumstance. Individuals within a population may evolve to be more similar to or more different from each other depending on the specific circumstances and selection pressures.

The four types of natural selection are as follows:

Stabilizing selection: This type eliminates extreme or unusual traits. Individuals with the most common traits are considered best adapted, which maintains the frequency of common traits in the population. Over time, nature selects against extreme variations of the trait. The size of human babies, for example, remains within a certain range due to stabilizing selection. Extremely small or extremely large babies are less likely to survive, so alleles that cause these extremes don’t last in the population.

Directional selection: In this type, traits at one end of a spectrum of traits are selected for, whereas traits at the other end of the spectrum are selected against. Over generations, the selected traits become common, and the other traits become more and more extreme until they’re eventually phased out. The biological evolution of horses is a good example of directional selection. Ancestral horse species were built for moving through wooded areas and were much smaller than modern day horses. Over time, as horses moved onto open grasslands, they evolved into much larger, long-legged animals.

Disruptive selection: In this type, the environment favors extreme or unusual traits and selects against the common traits. One example is the height of weeds in lawn grass compared with in the wild. In the wild, natural state, tall weeds compete for the resource of light better than short weeds. But in lawns, weeds have a better chance of surviving if they remain short because grass is kept short.

Sexual selection: Females increase the fitness of their offspring by choosing males with superior fitness; females are therefore concerned with quality. Males contribute most to the fitness of a species by maximizing the quantity of offspring they produce. Because males are concerned with quantity, competition between males for opportunities to mate exists in contests of strength. Therefore, structures and other traits that give a male an advantage in a contest of strength have evolved, including antlers, horns, and larger muscles. Because females choose their mates, males have also developed traits to attract females, such as certain mating behaviors and bright coloring.

Biological evolution happens to populations, not individuals. Individuals live or die and reproduce or don’t reproduce depending on their circumstances. But individuals themselves can’t evolve in response to a selection pressure. Imagine a giraffe whose neck isn’t quite long enough to reach the tastiest leaves at the top of the tree. That individual giraffe can’t suddenly grow its neck longer to reach the leaves. However, if another giraffe in the herd has a longer neck, gets more leaves, grows better, and makes more calves that inherit his long neck, then future generations of giraffes in that area may have longer necks.

The Evidence of Biological Evolution

Since Darwin first proposed his ideas about biological evolution and natural selection, many different lines of research from many different branches of science have produced evidence supporting his belief that biological evolution occurs in part due to natural selection.

Because a great amount of data supports the idea of biological evolution through natural selection, and because no scientific evidence has yet been found to prove this idea false, this idea is considered a scientific theory. (For more on the importance of theories in science, see Chapter 2.)

The following sections describe some of the evidence, both old and new, that supports Darwin’s theory and the tools modern scientists have used to obtain it.

Biochemistry

The fundamental biochemistry, the basic chemistry and processes that occur in cells, of all living things on Earth is incredibly similar, showing that all of Earth’s organisms share a common ancestry.

Case in point: All living things store their genetic material in DNA and build proteins out of the same 20 amino acids. Regardless of whether the organisms are flowers taking in carbon dioxide from the air, water from the soil, and light from the Sun; lions chomping down a wildebeest; or humans consuming a gourmet meal cooked by Wolfgang Puck himself, all organisms convert food sources to energy and store that energy in ATP. That stored energy is then used to power cellular processes such as the production of proteins, which is directed by the genes on strands of DNA.

Comparative anatomy

Comparative anatomy — which looks at the structures of different living things to determine relationships — has revealed that the various species on Earth evolved from common ancestors. Just like you have structural characteristics that are similar to those of your family members (think small ears, a large nose, and so on), structural similarities also exist between more distantly related groups.

As you can see in Figure 12-2, the skeletons of humans, cats, whales, and bats, for example, are amazingly similar even though these animals live unique lifestyles in very different environments. From the outside, the arm of a human, the front leg of a cat, the flipper of a whale, and the wing of a bat seem very different, but when you look at the bones within them, you see that they all contain the same ones — an upper “arm,” an elbow, a lower “arm,” and five “fingers.” The only differences in these bones are their size and shape. Scientists call similar structures such as these homologous structures (homo- means “same”). The best explanation for these homologous structures is that all four mammals are descended from the same ancestor — an idea that’s supported by the fossil record.

The homologous structures of mammals are particularly interesting in the case of whales because they reveal whales’ close relationship to land-dwelling animals. In fact, this evidence from comparative anatomy supports the idea that whales evolved from land-dwelling mammals into sea creatures.

Geographic distribution of species

How populations of species are distributed around the globe helps solidify Darwin’s theory of biological evolution. In fact, the science of biogeography, the study of living things around the globe, allows scientists to make testable predictions about biological evolution. Basically, if biological evolution is real, then you’d expect groups of organisms that are related to each other to be clustered near each other because related organisms come from the same common ancestor. (An exception to this prediction is that migratory animals could travel far from their relatives.) On the other hand, if biological evolution isn’t real, then there’s no reason for related groups of organisms to be found near each other. For example, a creator could scatter organisms randomly all over the planet, or groups of organisms could arise independently of other groups in whatever environments suited them best. When biogeographers compare the distribution of organisms living today, they find that species are distributed around the Earth in a pattern that reflects their genetic relationships to one another.

When Darwin compared the finches on the Galapagos Islands with those on mainland South America, the unique types of finches on the Galapagos Islands led him to hypothesize that the islands had been colonized by finches from the mainland. This hypothesis was later supported when modern scientists performed a genetic analysis of the Galapagos Islands’ finches and were able to demonstrate their relationship to each other and to their mainland ancestors.

Since Darwin’s time, many other examples have been found that illustrate how geographic distribution has influenced the biological evolution of organisms. The distribution of organisms on the Hawaiian Islands, for example, tells a very similar story to that of the Galapagos. Hawaii has types of living things that exist only on those islands but are related to living things found on the North and South American continents. The best explanation for the unusual life-forms found in Hawaii is that organisms arrived on the islands due to unusual events such as storms and then evolved separately from their mainland relatives.

Similarly, North and South America were separate continents before the Isthmus of Panama formed. Distinct groups of mammals lived in each area. Armadillos, porcupines, and opossums called South America home, whereas mountain lions, raccoons, and sloths lived in North America. The fossil record shows that these groups of mammals evolved separately until the Isthmus of Panama joined the two continents and the mammals were able to migrate back and forth.

Molecular biology

Molecular biology is the branch of biology that focuses on the structure and function of the molecules that make up cells. With it, biochemists have been able to compare the structures of proteins from many different species and use the similarities to create phylogenetic trees (they’re essentially family trees; see Chapter 10 for details) that show the proposed relationships between organisms based on similarities between their proteins.

With the development of DNA technology that allows for reading of the actual gene sequence in DNA (see Chapter 8 for more on DNA), modern scientists have also been able to compare gene sequences among species. Some proteins and gene sequences are similar between very distantly related organisms, indicating that they haven’t changed in millions of years; these sequences are called highly conserved sequences.

One of these highly conserved sequences produces a protein called cytochrome c, which is part of the electron transport chain that occurs in mitochondria. Humans and chimpanzees have exactly the same amino acid sequences in their cytochrome c proteins, which indicates that humans and chimpanzees branched off the trunk of the evolutionary tree very recently (“recently” in evolutionary terms is still quite a long time, about 6 million years in this case). The cytochrome c protein in rhesus monkeys differs from humans and chimpanzees by just 1 amino acid (out of a total of 104), indicating that rhesus monkeys are slightly more distantly related to humans.

Fossil record

The fossil record (all the fossils ever found and the information gained from them) shows detailed evidence of the changes in living things over time. During Darwin’s day, the science of paleontology, which studies prehistoric life through fossil evidence, was just being born. Since Darwin’s time, paleontologists have been busy filling in gaps left in the fossil record in order to explain the evolutionary history of organisms.

Hundreds of thousands of fossils have been found, showing the changing forms of organisms. For some types of living things, such as fish, amphibians, reptiles, and primates, the fossil record depiction of the changes from one form of the organism to another is so complete that it’s hard to say where one species ends and the next one begins.

Based on the fossil record, paleontologists have established a solid timeline of the appearance of different types of living things, beginning with the appearance of prokaryotic cells (see Chapter 4) and continuing through modern humans.

Observable data

Biological evolution can be measured by studying the results of scientific experiments that measure evolutionary changes in the populations of organisms that are alive today. In fact, you need only look in the newspaper or hop online to see evidence of biological evolution in action in the form of antibiotic-resistant bacteria.

In the 1940s, when people first started using antibiotics to treat infections, most strains of the bacterium Staphylococcus aureus (S. aureus) could be killed by penicillin. By using antibiotics, people applied a strong selection pressure to the populations of the S. aureus bacteria. The fittestS. aureus bacteria were those that could best withstand the penicillin. The bacteria that couldn’t withstand the penicillin died, and the resistant bacteria multiplied. Today, most populations of S. aureus are resistant to natural penicillin. Another strain of S. aureus called MRSA has evolved that’s not only resistant to natural penicillin but also to the semisynthetic methicillin, which used to be a great weapon in a doctor’s staph-infection-fighting arsenal. For most strains of MRSA, vancomycin is the last effective treatment, but for some new, highly dangerous strains, vancomycin is beginning to fail. In the late 1990s, the first strains of VRSA — vancomycin-resistant S. aureus — were reported. Doctors don’t currently have anything that can fight off VRSA; if a person gets a dangerous VRSA infection, chances are he’ll die.

Using antibiotics is a double-edged sword. By using them to fight infection, people get healthy but they also speed up the process of natural selection. The potentially good news is that because doctors and scientists understand biological evolution, they’re able to recognize what’s happening and can take action to counteract these trends (such as prescribing fewer antibiotics).

Radioisotope dating

Radioisotope dating indicates that the Earth is 4.5 billion years old — that’s plenty old enough to allow for the many changes in Earth’s species due to biological evolution. Isotopes are different forms of the atoms that make up matter on Earth (see Chapter 3 for more on isotopes). Some isotopes, called radioactive isotopes, discard particles over time and change into other elements. Scientists know the rate at which this radioactive decay occurs, so they can take rocks and analyze the elements within them. Using the known rates of radioactive decay and the types of elements that were originally present in the rocks, scientists can calculate how long the elements in a particular rock have been discarding particles — in other words, they can figure out the age of the rock (including rocks with fossils).

Why So Controversial? Evolution versus Creationism

Virtually all scientists today agree that biological evolution happens and that it explains many important observations about living things, but many nonscientists don’t believe in biological evolution and are often violently opposed to it. They prefer to take the Bible’s creation story literally. These wildly differing viewpoints have led to one of the great debates of all time: Which is correct, evolution or creationism? (Creationism is the idea that God created the world and all the life on it out of nothing. Most creationists believe the creation story that’s told in the Bible’s book of Genesis.)

The idea of biological evolution has inspired so much controversy over the years in large part because many people think it contradicts the Christian view of humanity’s place in God’s design. According to the ancient Greek philosopher Aristotle, no accidents occur in nature; therefore, everything in nature is created for a purpose. A 17th-century thinker, William Paley, built on this idea with his theory of intelligent design: Beautiful designs don’t arise by chance; if a beautiful design exists, the designer must also exist.

At the root of the controversy about biological evolution, then, seems to be this question: If living things developed in all their wonderful complexity due to natural processes and without the direct involvement of God, what does that do to man’s place in the world? Is mankind not “special” to God?

If you have strong religious beliefs and you think that accepting biological evolution as a fact would somehow make you less special to God, then it’s easy to see how belief in biological evolution creates conflict. But are biological evolution and religious faith necessarily in conflict? Many religious figures and scientists don’t think so. In fact, many scientists have strong religious beliefs, and many religious leaders have come forward to say that they believe in biological evolution.

Ultimately, each person’s beliefs are under his or her own control. But scientists stress the difference between beliefs, or faith, and science.

Science is an attempt to explain the natural world based on observations made with the five senses. Scientific ideas, or hypotheses, must be testable — able to be proven false — by observation and experimentation (see Chapter 2 for more on the nature of science).

The existence of God isn’t within the scientific realm. God is widely believed to be a supernatural being, outside the workings of the natural world. Belief in the existence of God is therefore a matter of faith.

Because intelligent design and creationism invoke the existence of a supernatural designer or creator, they’re neither scientific ideas nor scientific theories and can’t be tested or observed by scientific means. People who support intelligent design often support their arguments with observations of the natural world, but the explanations they propose for their observations aren’t based in the natural world, nor do they conduct experiments of their ideas based in the natural world.

Creationism and intelligent design don’t follow the fundamental rules of science and can’t be considered scientific ideas.

Table 12-1 puts the scientific and creationist arguments about biological evolution side by side so you can compare them and come to your own conclusions about what you believe.

How Humans Evolved

You’re a member of the Homo sapiens species. Humans are the only living species of hominids (modern humans and their extinct relatives) on the planet, but scientists have found fossils of other hominid species that give clues to our evolutionary origins.

The closest living relatives to humans are other primates, such as apes and chimpanzees. Primates are the order of mammals that includes monkeys, apes, and humans. They have large brains, grasping hands, and three-dimensional vision. (See Chapter 10 for the full scoop on the various categories in the taxonomic hierarchy.)

In the sections that follow, we tell you all about the tools scientists use to fill in the blanks about how humans evolved as well as the discoveries and connections they’ve made over the years.

Fossil finds

Perhaps the best clue to understanding why you have the physical structure you do is the fossil record of hominids. Scientists use fossils to piece together clues about where humans came from and what our relationship is to other primates. Hominid fossils are rare and often incomplete, but when new ones are found, they contribute new pieces of information to the story.

Scientific theories that are supported by lots of evidence from many lines of research, like the theory of biological evolution, don’t usually change substantially in response to new evidence. Instead, new evidence helps refine the theories and point out important details of how processes work.

Scientists’ ideas about the biological evolution of humans have developed over time. Following is a rundown of the different hominid fossil discoveries made since the late 17th century:

In 1891, researcher Eugene Dubois discovered a few bones in Java, Indonesia, a large island off the southeastern coast of Asia. Calling his discovery Java Man, Dubois thought he’d found the link between ape and man. What he found certainly was an ancestor to modern Homo sapiens, but it wasn’t apelike. Dubois had actually found a member of the species Homo erectus, one of the earliest walking hominids. Other Homo erectus bones have been found in China and Africa.

During the 1930s, a researcher named Raymond Dart examined a small skull that was found in Taung, a town in South Africa. After studying the bone structure and realizing that the skull contained a petrified brain, Dart came to the conclusion that the skull belonged to a child who was about 6 years old and a member of a human ancestral species. The remnants were called Taung Child. Dart thought he had found a missing link between apes and men, but others disagreed. Dart was ridiculed for suggesting that a human ancestor was “out of Africa,” when the thinking at that time was that the first human species came from Asia (due to the hullabaloo surrounding Java Man). But Dart persevered, even though his belief was unpopular at the time. He classified his skull as Homo habilis, meaning handyman, because crude stone tools were found near the bones.

In the 1930s, Louis and Mary Leakey began excavating the Olduvai Gorge in Tanzania, Africa. Three decades later, their son Richard noticed the jaw of a saber-toothed tiger sticking out of the archaeological site. Digging continued in the area, and eventually pieces of three skeletons were unearthed. The skeletons were also classified as those of Homo habilis and were dated at about 2 million years old. The Leakeys continued their work at Olduvai Gorge well into the 1980s, and in 1984, they unearthed a spectacular find: the first (and still the only) full skeleton of a Homo erectus, dated at 1.6 million years old.

In 1994, Richard Leakey’s wife, Meave, headed upstream from Olduvai Gorge to Kanapoi in northern Kenya and found a 4.2-million-year-old hominid ancestor. The lower jaw was complete, and the teeth were surprisingly like those of a modern human. However, the shape of the jaw was like that of a chimpanzee. Pieces of lower leg bone were also found, indicating that the creature walked on two legs. Meave named her find Australopithecus anamensis.

In the years since Dart and the Leakeys, several more fossilized bones have been discovered along the southeastern coast of Africa, giving Africa the nickname “the cradle of civilization.” In particular, a 3.2-million-year-old skeleton of the Australopithecus afarensis species was found in Ethiopia (part of northern Africa) in 1974 by Don Johanson; the skeleton was nicknamed Lucy.

Bones from a 4.4-million-year-old skeleton found in Ethiopia are currently being studied. The skeleton is called Ardipithecus ramidus, and because it’s the oldest known ancestral fossil, scientists are using it to try and determine whether this organism was in fact a direct ancestor to humans.

Scientists can use fossilized bones like those of early hominids such as Lucy to understand how our ancestors’ more apelike features evolved into the features of modern humans. Table 12-2 gives you an overview of the physical changes that occurred as apes evolved into humans.

Digging into DNA

The development of DNA technology has played a huge role in helping scientists read some of the human history encoded in DNA (we cover the complexities of DNA in Chapter 8). By simply comparing the DNA sequences of hominids, scientists can discover several pieces of information, such as

Which hominids are most closely related: Species that are closely related have greater similarities in their DNA sequences than species that are more distantly related. Humans are most closely related to chimpanzees; our DNA sequences are about 97 percent identical. Today, the current line of thinking is that there was an apelike species alive 10 to 20 million years ago that branched into a line of gorillas around 7 million years ago. That species then branched off into two lines about 5 to 6 million years ago. One of these branches on the family tree led to chimpanzees, and the other evolved into humans.

How hominids migrated: By comparing the genetic relationships between hominids, the age of certain fossils, and the geographic locations of these fossils, scientists can figure out where species originated and where they traveled.

When new species emerged: In animals, species are defined based on their ability to interbreed successfully. Typically, two organisms that can successfully produce offspring are considered to be in the same species, whereas organisms that can’t produce offspring together are considered to be unique species. With extinct species, scientists can’t make direct observations of who could interbreed. However, scientists can look at the DNA sequences from fossils to see who was mingling DNA. Scientists have compared DNA sequences from humans and other living primates, as well as those from fossilized hominids, including fossilized Neanderthals.

Check out the big brain on the Homo sapien

It’s one thing to know how Homo sapiens evolved into today’s modern humans. It’s another thing entirely to understand why these changes happened. For example, why did the human brain become so much larger than that of other hominids? The clues about why things happened the way they did are pretty scarce, but they do exist in the form of tools found with a skeleton, evidence of burial of human remains, and evidence of the use of fire. From these types of clues, scientists can put together hypotheses to explain the evolutionary pathway of modern humans. Some of these hypotheses are as follows:

As human ancestors began to walk upright, they soon began to hunt. Therefore, they went from being herbivores to carnivores. One factor that led to this development was climate change. As the Earth began to warm up, some of the forests disappeared and became open savannas. In an open savanna, it’s much easier to see prey (especially if you’re standing). So, human ancestors became successful hunters and ate plenty of meat.

Eating plenty of meat, with all the fats and proteins meat contains, made hominid brains bigger, and bigger brains were selected for over time. (However, scientists still don’t know why.)

As the shape and size of brains changed and enlarged, ancestral females had to give birth earlier so the offspring’s head could fit through the pelvic bones. Because babies were born earlier, they were much more dependent on their mothers for a longer period of time. This change meant that the mother couldn’t contribute to hunting, but she still needed adequate nutrition to make milk to breastfeed her baby. The father and other members of the clan therefore had to help the mother by bringing her food. The fact that the mother had to rely on others for her survival and that of her baby led to the formation of close ties with other members of the clan.

Table 12-3 compares the brains of different hominid species.