I asked Prof. Margulis if she had anything to say about Peter Ward's Medea hypothesis. She did not. Crispin Tickell, chairing, said it did not, in fact, contradict Gaia. 1. Redistribution of elements2. Salt: redistribution and retardation of its dissolution3. Augmentation of solid, liquid and gas interfaces4. Water retention and distribution on a planetary scale5. Production of granite (a mineral found nowhere else in the solar system, comprising 0.4% of the Earth lithosphere) 
Martin Brasier outlined key points from his book Darwin's Lost World, including the idea that the development of an anus (a through-gut and the capacity to digest in new ways) was a fundamental (my bad pun, not his) innovation enabling the Cambrian explosion.
He said his hunch was that the perturbations in the Earth system consequent upon human activities were so great that 'we could be on the cusp of a Cambrian-like transformation' of life on Earth (bigger than, say, the K-T) though whether it would be a 'new Cambrian explosion' or a 'return pre-Cambrian conditions' he was not, when I asked him, inclined to speculate.
Where the dust blows through these heights there once shone a silent sea. Footnotes
 Margulis spells out these questions and others more fully in a contribution to Scientists on Gaia (2004). See comment attached at the foot of this post.
 It seems to me, though, that there may be something to be said for Tyler Volk's view that 'Gaia is life in a wasteworld of byproducts' (dubbed 'Garbage Can Gaia' by Ward), and Prof. Basier hinted as much.
 The image is Suilven, an inselberg of Torridonian sandstone on top of Lewisian gneiss. The quote is from Cold Mountain. Brasier imagines Charles Lyell sketching three riddles after a walking on Quinag:
1. A mountain that stood on its head.
2. An ocean that disappeared.
3. A rock that swallowed time.
P.S. 8 July: The continents 'were green' in the Neoproterozoic, according to a Letter to Nature from L. Paul Knauth & Martin J. Kennedy. See also Dawn of the animals: Solving Darwin's dilemma.