The question arises: can a planet itself be considered alive? Lynn Margulis, a prominent figure in late 20th-century biology known for her unconventional ideas, believed it could. Along with chemist James Lovelock, they proposed that life significantly alters planetary conditions, blurring the lines between the living and nonliving aspects of Earth. Initially met with skepticism and branded as pseudoscience, their Gaia hypothesis is gaining traction due to emerging scientific evidence. This theory offers a fresh perspective on the interplay between life and planetary environments, fundamentally altering how we search for extraterrestrial life.

Margulis and Lovelock observed that Earth’s biosphere exhibits characteristics akin to those of a living organism. It maintains "homeostasis" through self-regulation, with vital elements such as atmospheric temperature, oxygen levels, and ocean chemistry being remarkably stable. They posited that the entirety of life interacts with its environment to sustain these global conditions, effectively viewing Earth as a living entity dubbed Gaia.

The duo challenged the conventional Darwinian view of evolution, which suggests that life merely adapts to changing environmental conditions. They argued that life actively influences its surroundings, reshaping the planet in the process. This paradigm shift allows us to recognize natural features like coral reefs and limestone cliffs as integral components of a vibrant, interconnected living system, rather than mere backdrops against which life unfolds.

Despite its growing acceptance, the Gaia hypothesis has faced obstacles. Critics have labeled it ambiguous and lacking a clear, testable scientific basis, complicating its evaluation. Margulis and Lovelock's blending of science with philosophy and their willingness to embrace controversy did not help their cause. Ultimately, Gaia should be viewed not merely as a hypothesis, but as a framework for understanding planetary life science—emphasizing that life is not a mere addition to a functioning Earth, but a crucial element of its evolution.

The rise of Earth system science has led to a deeper understanding of how profoundly life has transformed our planet. Observations from space reveal that Earth, unlike its seemingly barren counterparts, began a unique evolutionary journey once conditions allowed life to emerge. Advanced technologies have uncovered the extensive influence of life on Earth's biogeochemical cycles, affirming that the oxygen we breathe is a product of biological processes.

This intricate relationship extends to Earth's geology and mineral diversity, with many minerals being formed as a result of biological activity. Geochemists assert that most of the over 4,000 minerals on Earth owe their existence to life. This correlation suggests that signs of life on other planets might be inferred from their mineral compositions, similar to how we search for atmospheric gases indicative of biological processes.

Moreover, the dynamics of Earth's interior, traditionally seen as mechanical and independent of biology, are increasingly recognized as being influenced by life. The interplay between biological and geological systems may help maintain critical conditions for life, as seen through the lens of plate tectonics, which some argue has been shaped by living processes.

As we explore the cosmos, we recognize the potential for life on planets similar to Earth. However, it is crucial to understand that life may not be a transient occurrence but a planetary-scale phenomenon that can either establish or extinguish habitability. The unique qualities that have allowed life to flourish on Earth, such as continuous geological activity and a stable environment, may not be present in other worlds like Mars or Venus, where life may have failed to take root permanently.

Viewing planets as dynamic entities rather than static locations for life prompts a reevaluation of how we understand the origin of life. Life may not merely exist on a planet, but actively shape its identity and evolution, akin to a fire that once ignited becomes self-sustaining.

In this light, a "living worlds" perspective suggests that over time, planets either become devoid of life or, like Earth, are transformed by it. Once life gains a foothold, it may become deeply entrenched within planetary processes, rendering it nearly impossible to eliminate. This resilience reflects an extraordinary aspect of Earth's biosphere, which has survived through dramatic changes.

The emergence of industrial society has introduced new dynamics on Earth, represented by the bright lights observable from space. This phenomenon invites speculation about whether we are entering a new phase in planetary evolution, potentially redefining what it means for a planet to be alive.

The text draws from David Grinspoon's book Earth in Human Hands, which explores these profound themes.