Forty years ago Lovelock introduced the Gaia hypothesis and the criticisms by opponents and support by proponents are continuously developing. Currently, the extreme teleological version of Gaia has been ignored. The weak form of the Gaia hypothesis, which has been generally accepted, are the views that life collectively has a significant effect on earth's environment and the feedback coupling system of life and environment can shape their coevolution. However, three other Gaia propositions are strongly debated: the Homeostatic Gaia, the Optimal Gaia, and the Gaia based on natural selection. The Homeostatic Gaia states that biologically mediated feedbacks contribute to environmental homeostasis. Opponents argue this proposition contradicts ice core records and the findings of climatic feedback research. Proponents argue the homeostasis of the earth's biota-environment system results from a combination of positive and negative feedbacks. The primary concern of the Gaia hypothesis is focused on billions years of the earth history. Although the Gaia hypothesis can be falsified in relatively short timescales, that does not indicate it can be falsified in longer timescales. The Optimal Gaia states that life regulates the physical and chemical environment to meet the biosphere's needs. Although a conclusion has not been formed regarding alteration of or adaption to the environment, proponents assert that when the biota-environment system is perturbed or collapses, the dominant effect will appear. A system with strong environmental feedback will be prone to rapid transitions between states, whereas one where adaptation dominates will change more slowly. Opponents agree life can benefit itself by the biologically mediated effect on the environmental condition, but they claim life should firstly adapt to the environmental condition by natural selection. It is very likely the earth evolved to this environmental condition only by chance. The last proposition, Gaia based on natural selection, specifies that biologically medicated feedbacks come from Darwinian natural selection. The opponents argue the statement that life-enhancing effects are favored by natural selection, but this is not always valid, except when a heritage trait confers a reproductive advantage to its carrier. Natural selection is a mechanism, not a principle. Proponents believe that natural selection is not an essential condition to environmental regulation. Natural selection based on by-products can resolve the cheat issue in interspecific cooperation raised by many evolutionists. Proponents acknowledge natural selection does not always favor life-enhancing effects, however, when a heritage trait benefits its carriers compared to non-carriers, natural selection can confer an evolutionary advantage to carriers. Disputes continue, but as a hypothesis generator, the Gaia hypothesis has proven its worth. Since the industrial revolution, the impact of human activities on the biosphere has been constantly enhanced. Under this situation, the Gaia hypothesis must incorporate human activities or it will decline and be substituted by other theories or hypotheses. In the future, scientists researching the Gaia hypothesis should strive to explore the general principle, which can apply to the whole biosphere and insist on systematic thinking. In terms of specific methods, consideration to experimenting with biosphere-scale metrics, such as GPP and cycling ratio is recommended. Scientists must build new mathematic models and particularly new mechanic models with regards to biogeochemistry. Costs and benefits need to be addressed on different scales when discussing life benefiting the environment.