Homologies: comparative anatomy
Organisms that are closely related to one another share many anatomical similarities. Sometimes the similarities are conspicuous, as between
crocodiles and alligators, but in other cases considerable study is needed for a full appreciation of relationships.
Modification of the tetrapod skeleton
Whales and hummingbirds have tetrapod skeletons inherited from a common ancestor. Their bodies have been modified and parts have been
lost through natural selection, resulting in adaptation to their respective lifestyles over millions of years. On the surface, these animals look
very different, but the relationship between them is easy to demonstrate. Except for those bones that have been lost over time, nearly every
bone in each corresponds to an equivalent bone in the other.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_06 2/27/11
In your journal: 1) What are some similar bones between baleen whales and
hummingbirds?
2) Why would such different animals as whales and hummingbirds share so many
bone structures? What does this say about the idea that they share common
ancestors?
Homologies: cellular/molecular evidence
All living things are fundamentally alike. At the cellular and molecular level living things are remarkably similar to each other. These
fundamental similarities are most easily explained by evolutionary theory: life shares a common ancestor.
The cellular level
All organisms are made of cells, which consist of membranes filled with water containing genetic material, proteins, lipids, carbohydrates,
salts and other substances. The cells of most living things use sugar for fuel while producing proteins as building blocks and messengers.
Notice the similarity between the typical animal and plant cells pictured below — only three structures are unique to one or the other.
The molecular level
Different species share genetic homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans.
These genes are slightly different in each species, but their striking similarites nevertheless reveal their common ancestry. In fact, the DNA
code itself is a homology that links all life on Earth to a common ancestor. DNA and RNA possess a simple four-base code that provides the
recipe for all living things. In some cases, if we were to transfer genetic material from the cell of one living thing to the cell of another, the
recipient would follow the new instructions as if they were its own.
These characteristics of life demonstrate the fundamental sameness of all living things on Earth and serve as the basis of today's efforts at
genetic engineering.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_08 2/27/11
In your journal: 1) What are some similar cell structures between animal and plant
cells?
2) Why would such different things as plant and animal cells share so many? What
does this say about the idea that they share common ancestors?
Tracing Similarities And Differences In Our DNA
What percent of their genes match yours?
1
2
3
4
Another human?
100% - All humans have the same genes, but some of these genes contain
sequence differences that make each person unique.
A chimpanzee?
98% - Chimpanzees are the closest living species to humans.
A mouse?
92% - All mammals are quite similar genetically.
A fruit fly?
44% - Studies of fruit flies have shown how shared genes govern the growth and
structure of both insects and mammals.
Yeast?
26% - Yeasts are single-celled organisms, but they have many housekeeping genes
that are the same as the genes in humans, such as those that enable energy to be
derived from the breakdown of sugars.
A weed (thale cress)?
18% - Plants have many metabolic differences from humans. For example, they
use sunlight to convert carbon dioxide gas to sugars. But they also have
similarities in their housekeeping genes.
Why Were Genes Used In This Comparison, and How Do They Relate To DNA?
Genes are the fundamental units of DNA function. In DNA terms, genes are discrete sections of the DNA sequence that are
part of much longer DNA molecules. They provide the biochemical instructions for producing all of the components of
biological organisms. Some genes specify visible physical traits, while others govern metabolic processes. Most traits, such as
the shape of your face, require the actions of many genes.
Why Are We So Similar?
The DNA of these species is so similar because the basic organization of life is widely shared, with the largest differences
found between plants and animals, or between tiny single-celled organisms like yeast and large multicellular organisms like
ourselves. The similarities reflect a common ancestry that appears to be shared by all life on Earth.
Are People Really Identical?
Even though humans share 100% of the same genes, the instructions contained within the genes are not entirely identical.
Each person is unique. People have different hair colors, facial structures, and other traits. These differences between
individuals result from very small differences in their DNA sequences. DNA also contains many so-called "housekeeping
genes" that control important metabolic processes. As you will see, some of the differences in these genes can cause illness.
Although the DNA of any two people on Earth is, in fact, 99.9% identical, even a tiny difference can have a big effect if this
difference is located in a critical gene.
http://www.koshland-science-museum.org/exhibitdna/intro03.jsp 2/27/11
In Journal: 1) Look for patterns in the data above and write down any you notice.
2) Why would these organisms share DNA? What does this say about the idea that we
share common ancestors?
Homologies: anatomy
Individual organisms contain, within their bodies, abundant evidence of their histories. The existence of these
features is best explained by evolution.

Several animals, including pigs, cattle, deer, and dogs have reduced, nonfunctional digits, referred to as
dewclaws. The foot of the pig has lost digit 1 completely, digits 2 and 5 have been greatly reduced, and
only digits 3 and 4 support the body. Evolution best explains such vestigial features. They are the
remnants of ancestors with a larger number of functional digits.

People (and apes) have chests that are broader than they are deep, with the shoulder blades flat in back. This is because we, like
apes, are descended from an ancestor who was able to suspend itself using the upper limbs. On the other hand, monkeys and other
quadrupeds have a different form of locomotion. Quadrupeds have narrow, deep chests with shoulder blades on the sides.

Hoatzin chicks have claws on their wings, as do some chickens and ostriches. This reflects the fact that bird ancestors had clawed
hands.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_05 2/27/11
In your journal: 1) What are some similar bones amongst different organisms?
2) Why would such different animals that they share so many bone structures? What
does this say about the idea that they share common ancestors?