Summary: Extraterrestrial Life: Problem Set #3
Due, in class, Thursday February 28th
1) Outline the key properties that seem to be common to all life on Earth.
Explain why the existence of these shared features supports the idea that
all life on Earth arose from a common ancestor.
(lecture #9) Properties common to all life on Earth include the fact that life
is contained within cells, relies on proteins made out of amino acids,
utilizes DNA as the carrier of hereditary information, and makes use of
ATP as an energy carrier. The fact that all life shares these features in
particular the "non-obvious" ones such as the fact that the same fairly
small set of amino acids is used by almost all life forms seems very
unlikely to be a chance occurrence (i.e. it seems improbable that if life
started independently in two different places, everything on this list would
be the same). Rather it seems likely that all life today was descended from
a common ancestor which already possessed these features.
2) Suppose that life is discovered on Mars. Explain how we might be able to
determine whether it shares a common ancestor with life on Earth. How
would the prospects for making such a determination be affected if all that
is found on Mars are fossils of life that died out hundreds of millions of