Common Ancestry

Darwin and Evolution

For centuries, scientists have been posing questions about the origin of life on Earth. In 1859, the English naturalist Charles Darwin published On the Origin of Species, which provided sound logic and evidence about how organisms change over time. Darwin proposed a theory that all forms of life developed gradually from different, and often much simpler, parent organisms. He proposed that weaker, or less adaptable, members of a population died without reproducing and that the strongest members of the population passed on traits to the next generation.

Scientists have since confirmed that each population of organisms undergoes changes over time, and this process is what scientists call evolution. Within a population, a small group of individuals can become isolated from the rest · of the population. Over many generations, the isolated group may evolve different traits than those within the original population of organisms. If an isolated group becomes so different that it is no longer able to reproduce with the original population, it is then considered a new species. A species is a distinct group of organisms that can reproduce only with each other to produce live offspring. These ideas all became part of Darwin’s famous theory of evolution by natural selection.


Scientists are able to find evidence of evolution when examining fossils. A fossil is the preserved remains of an organism that was once living. When an organism dies, the soft parts of it decay, but the harder parts are left behind. Over many years, layers of sediment build up over the dead organism. As sediment builds up, pressure, along with chemical changes, causes the sediment to harden into rock. This type of rock forms layers that represent different geologic time periods and is known as sedimentary rock.


Other fossils formed when insects or small animals were trapped in tree sap or ice. A fossil can be part of the organism itself, such as bones, teeth, or shells, or it can also be a trace of an organism, such as an imprint of a leaf or a footprint. A fossil can even be something left behind by an organism, such as animal droppings.

Fossils provide a snapshot of life on Earth over time, and they reveal that life on Earth has changed over time. Fossil evidence showed scientists that new species appeared at different points in time, which strongly supported Darwin’s theory of evolution.

Think about Science

Directions: Answer the following questions.

  1. Two organisms that can produce live offspring are considered to be the same [ blank ].
  2. List two species that are similar and that you think may have evolved from a single species into two distinct species.

The Theory of Universal Common Ancestry

In Darwin’s publication On the Origin of Species, he also proposed that all organisms that have ever lived on Earth descended from a single primitive ancestor. An ancestor is an organism that lived in the past and produced offspring. These offspring produced another generation of offspring, and this continues with every new generation. When we look at the ancestry of a person or an organism, we look at the whole line of descendants coming from a single parent. Thus, Darwin’s theory became known as the Theory of Universal Common Ancestry (UCA). This theory was difficult for people to accept because it suggested that an oak tree, a caterpillar, a dog, and a human all originated from a common ancestor. During Darwin’s lifetime, there was no clear way to test his theory.

It is easy to see how people in the same family are related or even how two different dog breeds might be related, but it is very difficult to see evidence that a human is related to bacteria. Long after Darwin’s UCA theory was proposed, scientists developed a quantitative large-scale test for the relatedness of species. With this test, they have found that evidence in the natural world overwhelmingly supports UCA. Scientific evidence links all three domains of life: Eukarya (humans, animals, plants, yeast), Bacteria, and Archaea. Scientists can now link every known organism back to a single common ancestor. However, what this universal ancestor looks like and where it lived are still questions unanswered by scientists. The most current hypothesis is that the first form of life on Earth was a simple, single-celled organism that lived in water.

Think about Science

Directions: Answer the following questions.

  1. The theory of Universal Common Ancestry states that A. all humans are related to eac.h other. B. all life originated from a single organism. C. all life has slowly changed over time. D. all species that look similar are related to each other.
  2. What evidence has scientists found to support UCA?


The evolutionary relationship of organisms can be studied using a method called cladistics. Cladistics is the systematic method for making and testing predictions about evolutionary relationships among organisms. Organisms are organized into groups, or clades, and every member of a clade shares the same evolutionary history. Therefore species in a clade have more in common with each other than they do with species of another clade. Also, the members of a clade share a unique group of characteristics that did not exist among their distant ancestors. This type of categorization of organisms assumes that new species arise when an existing species is split into two or more isolated groups. It also assumes that all organisms continue to change over time.

Scientist can describe the evolutionary relationship between species by using a diagram. These diagrams, called cladograms, show where traits diverge over time. To build a cladogram for a group of species, you need to rely on research and evidence collected by scientists about each species.

To construct a cladogram, follow four simple steps:

  • First, choose a group of organisms to compare. As an example, let’s look at the relationship between the squid, the frog, the carp, the eagle, the whale, and the cow.
  • Next, construct a table with the name of each organism across the top and some structural features you want to compare listed in the first column.

Cladograms Data

  • After the table is complete, group together the organisms that share the same traits within concentric circles as shown.

Cladograms 1

  • Finally, now that you have an idea about how these species are the same and how they are different, draw a cladogram as follows.

Cladograms 2

All the organisms that branch off to the right of a trait have that trait, and those that lie to the left of a trait do not have that trait. In this way, you can see where species diverge from one another. You can also see which species are more closely related to each other.