Biology and Xenobiology Links
To'ul'h vs Human
Image from Andrew P.

Biology on Earth and speculations about Alien Life

The Limits of Organic Life in Planetary Systems (Online Book)

Biology Concepts

Palaeos (M. Alan Kazlev was an early contributor to this site)

Tree of Life

University of California Museum of Paleontology

Rare Earth Hypothesis

Astrobiology Magazine

Fermi Paradox (Wiki)