Andrew H. Knoll — American Scientist

Andrew Herbert Knoll is the Fisher Professor of Natural History and a Professor of Earth and Planetary Sciences at Harvard University. Born in West Reading, Pennsylvania in 1951, Knoll graduated from Lehigh University with a bachelor of arts in 1973 and received his Ph.D. from Harvard University in 1977 for a dissertation entitled "Studies in Archean and Early Proterozoic Paleontology." Knoll taught at Oberlin College for five years before returning to Harvard as a professor in 1982. At Harvard, he serves in the departments of Organismic and Evolutionary Biology and Earth and Planetary Sciences... (wikipedia)

If you look at the ecological circuitry of this planet, the ways in which materials like carbon or sulfur or phosphorous or nitrogen get cycled in ways that makes them available for our biology, the organisms that do the heavy lifting are bacteria.
It's fair to say when you go out and walk in the woods or on a beach, the most conspicuous forms of life you will see are plants and animals, and certainly there's a huge diversity of those types of organisms, perhaps 10 million animal species and several hundred thousand plant species.
I think you can say that life is a system in which proteins and nucleic acids interact in ways that allow the structure to grow and reproduce. It's that growth and reproduction, the ability to make more of yourself, that's important.