What evidence points to a common source for all life on this planet? What is DNA?

Ignoring software-based life,

All extant life appears to share a single common ancestor.

Cellularity. Extant life is made up of a cell or cells. These living cells have phospholipid bilayer membranes, with embedded proteins to "add back" selective permeability.

Metabolism: All living cells perform many similar metabolic functions.

Genetic code: Information is stored as sequences of nucleobases along double-stranded DNA. This information is transcribed to RNA, and translated to polypeptide sequences. Polypeptides are processed and folded to become proteins. The information on the RNA can be divided into "codons" that specify a single amino acid in the polypeptide and with only minor variations codons are the same for all extant life.

> All extant life appears to share a single common ancestor.

Note the word "appears." What I've written here could also be interpreted as supporting a "single common Creator."

As Emucompboy says Cellularity, Metabolism, and the Genetic code all point to a common source. He thinks this points to a universal common ancestor, I think it points to a common creator.

This last universal common ancestor is generally agreed to be a simple single celled microbe. There is a million fold difference between the genome of a human and the simplist microbe living today so unless evolution can explain how new kinds (species in Darwin's usage) arise and how new genetic information is created and added to the genome it remains an implausible explanation.

So far all that has been directly observed is adaptation (micro-evolution, a change in allele frequency in a population over time), and devolution (loss of genetic information). Macroevolution (changing of one kind into another; esp. more advanced), or increases in genetic information have never been observed.

These observations, or lack of them, are more consistent with creation than evolution.

Fossil records show the first life formed around 3.5 billion years ago, and its believed that all life descended from a last universal common ancestor. If you look at geological history you can see how life diversified from a single organism during deep time. Also, every organism shares some genetic similarity with all other organisms.

What is DNA? In short, its a molecule with the ability to code for information in the form of base pair sequences. It consists of a phosphate, a sugar backbone (deoxyribose) and a nitrogenous base (A,T, G or C).