succinctly defined the hypothesis as follows: "The notion of the biosphere as an active adaptive control system able to maintain the Earth in homeostasis we are calling the 'Gaia hypothesis.'" Like other early presentations of Lovelock's idea, the Lovelock-Margulis 1974 paper seemed to give living organisms complete agency in creating planetary self-regulation, whereas later, as the idea matured, this planetary-scale self-regulation was recognized as an emergent property of the Earth system, life and its physical environment taken together. When climatologist Stephen Schneider convened the 1989 American Geophysical Union Chapman Conference around the issue of Gaia, the idea of "strong Gaia" and "weak Gaia" was introduced by James Kirchner, after which Margulis was sometimes associated with the idea of "weak Gaia", incorrectly (her essay "Gaia is a Tough Bitch" dates from 1995 – and it stated her own distinction from Lovelock as she saw it, which was primarily that she did not like the metaphor of Earth as a single organism, because, she said, "No organism eats its own waste"). In her 1998 book Symbiotic Planet, Margulis explored the relationship between Gaia and her work on symbiosis. Five kingdoms of life In 1969, life on earth was classified into five kingdoms, as introduced by Robert Whittaker. Margulis became the most important supporter, as well as critic – while supporting parts, she was the first to recognize the limitations of Whittaker's classification of microbes. But later discoveries of new organisms, such as archaea, and emergence of molecular taxonomy challenged the concept. By the mid-2000s, most scientists began to agree that there are more than five kingdoms. Margulis became the most important defender of the five kingdom classification. She rejected the three-domain system introduced by Carl Woese in 1990, which gained wide acceptance. She introduced a modified classification by which all life forms, including the newly discovered, could be integrated into the classical five kingdoms. According to Margulis, the main problem, archaea, falls under the kingdom Prokaryotae alongside bacteria (in contrast to the three-domain system, which treats archaea as a higher taxon than kingdom, or the six-kingdom system, which holds that it is a separate kingdom). Margulis' concept is given in detail in her book Five Kingdoms, written with Karlene V. Schwartz. It has been suggested that it is mainly because of Margulis that the five-kingdom system survives. Controversies It has been suggested that initial rejection of Margulis' work on the endosymbiotic theory,