It's time we freed Gaia from the New Age cell where she's been imprisoned since the Seventies.
As James Lovelock said, Gaia is Earth seen as a single physiological system, an entity that is alive at least to the extent that, like other living organisms, its chemistry and temperature are self-regulated at a state favorable to life. It is a whole system, not arbitrarily divided into biosphere, atmosphere etc.
Lovelock was not a mystic, he was a British chemist, hired in 1963 by NASA to investigate through spectrographic analysis of the atmosphere of Mars, whether that planet would support life. To provide data points for comparison, Lovelock analyzed the atmospheres of Venus and Earth as well. Abiological Earth is what one would interpolate, on the continuum of planetary proximity to our sun. He was not looking for Gaia, but there she was, irrefutably present in his data.
Here's what he found:
Component Mars Abiological Earth Earth Venus
CO2 95% 98% .00033 98%
Nitrogen 2.7% 3% 77% 3-4%
Oxygen trace trace 21% trace
Surface temp -53 C 290 C 13-15 C 477 C
Barometric Pres. .064 bars 60 bars 1 bar 90 bars
(Source: J. E. Lovelock (1979). Gaia: A new Look at Life on Earth. Oxford University Press, Oxford, U.K.) Read more at http://www.jameslovelock.org/page19.html
That's the chemical part. Lynn Margulis, a microbiologist, provided the Life part of the equation. She posited that bacteria were the first, simplest organisms to evolve. Cyanobacteria developed the ability to photosynthesize (to make food from sunlight). Through a process she called endosymbiosis, micro-organisms, instead of ingesting other micro-organisms, began to combine, forming larger more complex organisms. A cell is a community of micro-organisms. Read more at http://evolution.berkeley.edu/evolibrary/article/history_24
Gaia is described as symbiosis as seen from space - the same process occurring within organisms is also taking place on a planetary scale.
Neither Lovelock nor Margulis approached the question of consciousness, either in regard to our own species, nor to any other. But as research continues to find awareness of pain and pleasure, a sense of time, tool use, and planning among species besides our own, I can't help thinking that, as some religious and mystical traditions have long asserted, life has consciousness. And Earth is alive.
Caveat: In summarizing much larger amounts of information, I may have introduced inaccuracies. Please visit the cited sources to learn in depth about ideas I have touched on here.