Exactly when was life on Earth an Environmental Utopia, when mankind enjoyed Gaia in all her grandeur?

Can't speak for liberals, but an "environmental utopia" can be found historically over 10,000 years ago, prior to the advent of agriculture and subsequent civilization. Though 35,000 years or so would be more ecologically sound, as it was about that time that symbolism began taking root, laying the foundations for a society that is alienated from the natural world.