Understanding our past: DNA

The chimpanzee and another ape, the bonobo, are humans' closest living relatives. These three species look alike in many ways, both in body and behavior. But for a clear understanding of how closely they are related, scientists compare their DNA, an essential molecule that's the instruction manual for building each species. Humans and chimps share a surprising 98.8 percent of their DNA. How can we be so similar—and yet so different?

Human and chimp DNA is so similar because the two species are so closely related. Humans, chimps and bonobos descended from a single ancestor species that lived six or seven million years ago. As humans and chimps gradually evolved from a common ancestor, their DNA, passed from generation to generation, changed too. In fact, many of these DNA changes led to differences between human and chimp appearance and behavior.

If human and chimp DNA is 98.8 percent the same, why are we so different? Numbers tell part of the story. Each human cell contains roughly three billion base pairs, or bits of information. Just 1.2 percent of that equals about 35 million differences. Some of these have a big impact, others don't. And even two identical stretches of DNA can work differently—they can be "turned on" in different amounts, in different places or at different times.

Although humans and chimps have many identical genes, they often use them in different ways. A gene's activity, or expression, can be turned up or down like the volume on a radio. So the same gene can be turned up high in humans, but very low in chimps.

The same genes are expressed in the same brain regions in human, chimp and gorilla, but in different amounts. Thousands of differences like these affect brain development and function, and help explain why the human brain is larger and smarter.

The chimpanzee immune system is surprisingly similar to ours—most viruses that cause diseases like AIDS and hepatitis can infect chimpanzees too. But chimps don't get infected by the malaria parasite Plasmodium falciparum, which a mosquito can transmit through its bite into human blood. A small DNA difference makes human red blood cells vulnerable to this parasite, while chimp blood cells are resistant.

Every living thing on Earth shares the most fundamental structure of life: deoxyribonucleic acid, or DNA. Packed inside virtually every cell, DNA carries the genetic information needed to build and maintain an organism. Whenever living things reproduce, that DNA instruction book is passed on to the next generation.

Cell: A single human eye has hundreds of millions of cells—microscopic units of life. The cell's nucleus contains nearly all of its DNA.

Chromosome: Chromosomes are bundles of DNA that carry genetic information from parents to offspring. Humans have 23 pairs of chromosomes, inheriting half of each pair from each parent.

DNA Base Pairs: DNA is made up of two very long strands twisted together. DNA carries information encoded in a long series of four chemicals: adenine, thymine, cytosine and guanine, or A, T, C and G.

Genes: Characteristics ranging from hair color to health are influenced by particular segments of DNA. Together these segments, called genes, control how our bodies look and function.

mRNA: In a multi-step process, genes tell our bodies how to develop and function. First, the gene is used as a template to make a messenger RNA (mRNA).

Ribsosome: Next, mRNA carries the gene's information outside the nucleus to the ribosome, or protein factory.

Protein: Finally, the ribosome translates the genetic code and builds a protein. Each protein does a specific task in the body, such as growing bone or fighting infections.

Cells are the tiny building blocks that make up all living things. And inside each of the 100 trillion cells that make up the human body is DNA.

Look at this human liver cell. Can you find the two different parts of the cell that house its DNA? Most of the DNA is in the nucleus. The rest is found in the thousands of tiny mitochondria, the cell's energy generators.

In a real cell, the DNA stays packed inside—it doesn't spring out and grow larger, as it does in this model!

All organisms pass copies of their DNA to their offspring. Occasionally, mistakes are made in the copying process, and future generations inherit those mistakes. So human DNA today carries a record of mistakes, or mutations, that happened in our ancestors.

Differences between human DNA and DNA from another species can help biologists estimate when the two species branched apart on the evolutionary tree. (An evolutionary trees is a representation of how a specific taxonomic group evolved new species over time. All trees are hypotheses, and are based on comparison of living species, fossils, and genetic data.) And comparing DNA from different groups of living humans reveals the history of ancient human migrations.

You inherit half of your DNA from your mother and half from your father. Before the DNA is passed to the next generation, it gets recombined, or shuffled—that's what makes each person unique. But two chunks of DNA—mitochondrial DNA and the Y chromosome—break the rule. They never get shuffled, so from generation to generation they remain unchanged. As a result, scientists can use them to look millions of years into the past.

Mitochondrial DNA passes from a mother to all her children, but only the daughters pass it to the next generation. Experts can trace female ancestry by studying patterns of mutations in mitochondrial DNA. Some of the mutations happened millions of years ago, others more recently.

The Y chromosome passes only from father to son; females don't have a Y chromosome. Scientists use samples of Y-chromosome DNA to construct a lineage showing relationships among groups of people from all parts of the world.

Scientists use DNA to reconstruct events in human evolution and human migrations. Dates from fossils of human ancestors help to confirm these findings.

By comparing DNA sequences from humans and chimpanzees, experts calculated that the last human-chimp ancestor lived roughly six million years ago. Later, the discovery of a hominid fossil dating back six to seven million years supported this claim.

To construct a single family tree connecting all humans, researchers analyzed mitochondrial DNA from people all over the world. They traced the root of that tree to a female lineage, nicknamed "mitochondrial Eve," about 150,000 years ago. All other female lineages alive at that time have died out.

Khoisan people in southern Africa carry the most ancient DNA mutations found in humans today.

If the human fossil record goes back millions of years, why don't we have DNA from all extinct hominid species? When animals die and decompose, chemical reactions break down the DNA inside their cells. Minerals gradually replace any remaining bone, and after 100,000 years or so the DNA.