READ: Gallery — What is Life?

In this image, a monarch butterfly in its caterpillar stage eats a milkweed leaf. The leaf processes energy from the sun and grows in a certain size and shape that distinguishes it as milkweed. The caterpillar, Danaidae plexippus, consumes the plant material (milkweed is its favorite), digesting it and turning it into the energy that will eventually fuel its transformation first into a chrysalis and then into a gold and black butterfly.

Charles Darwin was a naturalist and an author. His observations of diverse life forms and the way in which they adapted to their surrounding environments led him to develop his theory of natural selection. In 1859 Darwin published On the Origin of Species, introducing his theory on evolution and forever changing the science of biology. This illustration was used as the cover for an early edition of the book.

Artificial selection in the form of animal and plant breeding has been going on for thousands of years and Darwin's knowledge of it helped him to shape his theory of natural selection. Darwin knew that people had bred animals to favor certain traits and had "cultivated" vegetables or grains to make them more suitable as a food source. This picture shows different varieties of carrots, but color certainly isn't the only trait that can be artificially selected for. Animal breeders will often mate certain individuals so their offspring will carry desired traits forward. Modern understanding of DNA and new "genetic engineering"  techniques have produced new possibilities for artificial selection and new questions about life.

A living organism's ability to protect itself from its predators is one of the keys to its survival. In this picture, a flounder blends almost perfectly with the seafloor. Traits such as color are inherited from one generation to the next and, over many generations, certain colors or color patterns will be "naturally selected" by the survival rates of the fish that carry those traits. The same process of natural selection occurs with many other traits such as size, structure and specific internal functions.

The double helix structure of DNA, deoxyribonucleic acid, was discovered in 1953 in one of the great scientific races of the modern era. DNA is found in all life on Earth and contains the instructions that cells use to manufacture the materials that make up living organisms. The discovery of DNA and its structure was a major step in understanding the complexity of life and has led to new approaches for understanding and classifying life. Genome sequencing is a technique used by modern biologists to distinguish differences between and among various species.

DNA consists off four main "bases" connecting two nucleotide strands in a double helix structure. The four bases pair up: adenine(A) with thymine(T) and guanine(G) with cytosine(C) to connect the two strands, forming a spiral ladder. Different sequences of these four bases spell out individualized genetic instructions for different living organisms. Inside cells, the long DNA sequences are folded and packed into chromosomes, dense "manuals" of genetic information.

In human cells, the long sequences of DNA contained in a chromosome are packed inside the nucleus of the cell, protecting this important genetic information. DNA is so tightly wound within a chromosome that the 51 million to 245 million base pairs in one human chromosome are estimated to be up to two meters long if unraveled. This is an incredibly long length when you consider the microscopic size of the cell and the even tinier nucleus housing multiple chromosomes.

In this 1879 illustration, German naturalist and artist Ernst Haeckel demonstrates a predominant view of his time, that humans somehow represent the pinnacle of evolution. As biologists continue to study life on Earth and to gather more information, the way that the tree of life and human evolution is depicted continues to change.

In this Big History Project depiction of the tree of life, numerous branches of evolution are placed into a time context. An image of the entire tree of life would contain many more branches and would not be as focused on the history of humans.