All organisms are related by common descent from a single origin of life. They contain genetic information that encodes how they look and how they function. They also reproduce, extract energy from their environment, use energy to do biological work, synthesize complex molecules to build biological structures, regulate their internal environment, and interact with one another. Evolution has produced an enormous diversity of organisms that make up communities and ecosystems. Genomics helps scientists establish the evolutionary relationships between organisms.

Question 1

The origin of photosynthesis resulted in a gradual build-up of oxygen in the Earth’s atmosphere. Before the Earth had an oxygen-rich atmosphere, UV radiation on the Earth’s surface was so intense that it killed any organisms on land; life could only survive if it was protected from UV radiation by water. But as O₂ accumulated in the atmosphere, O₂ molecules began reacting with one another to form ozone (O₃). A layer of ozone gradually built up in the high atmosphere, and by about 500 million years ago it was sufficient to block enough UV radiation that life could colonize land.

Question 2

Among the list of common characteristics of life on Earth, there are some features that we might expect to be general to any origin of life (such as the need to extract energy from the environment and use it to do work), but other features that we expect to be unique to each origin of life. For example, although another origin of life might use a genetic information system of some kind, there is no reason that we would expect the details of how that system functioned to be the same. Another origin of life might well use something other than DNA, or use different nucleotides to make up DNA. Moreover, even if another origin of life used the same basic building blocks to make DNA, the genetic code that specified which combinations of nucleotides encode which amino acids in proteins would be expected to be different. Furthermore, it is unlikely that the same set of amino acids would be used to construct proteins. It is the commonalities of these details across life on Earth that allow us to conclude that all life on Earth has a single common origin.

Question 3

Fish typically have eyes that are used for sight, and eyes obviously require light to function. In a normal surface population, any mutation that disrupts eye-sight would be strongly selected against, because fish with non-functional eyes would be at a disadvantage compared to fish with functional eyes. Such mutations occur, but they are quickly eliminated from a surface population through natural selection. But in a cave population, there is no selection for eye-sight, so such mutations are not selected against, and would accumulate in a cave population. In the absence of selection for eye function, we would expect gradual loss of a complex structure like an eye. In fact, many species of cave organisms do gradually evolve eyelessness over time. But how do they compensate for the loss of sight? Usually, there is strong selection for other sensory systems that do not require light, such as cells that detect small vibrations, or chemical scents. The important point is that in the dark environment, selection conditions change, and some features are expected to be lost, and others gained, because different features are favored under natural selection in the new environment.

Question 4

Biologists use quantitative measures of similarities and differences between specimens to establish the relatedness of different species. The more similar, the more likely they have a recent common ancestor and the more different, the more likely their common ancestor is more remote in evolutionary time. Knowledge about phylogenetic relationships is also obtained from the fossil record and more recently from genomic analysis.