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the planetary machinery set our planet apart from all others known so far. It was not always so. On the early Earth, life was not part of the planetary machinery. The first billion years after the planet formed from swirling gases was barren of life. But during that time, the building blocks essential for life somehow appeared. Bombarding comets and meteorites may have brought carbon, water, nitrogen, and maybe phosphorus to the Earth. Or perhaps these building blocks were already present in the early lifeless Earth. The road from these building blocks to the array of life on Earth today was long and winding. In essence, a series of ratchets, hatchets, and pivots over geologic time guided a lifeless planet to one with staggering diversity. And each time, the complexityratcheted up a notch.
    The first pivot occurred around 3.5 billion years ago, when self-replicating single-celled organisms arose from the primordial stew of the building blocks and theprecursors for life. Some of these simplest forms of life lived in rotten-smelling sulfur swamps, others in deep-sea vents where they could use energy from heat as it escapedfrom the Earth’s interior. For 2 billion years, these simple purple and green bacteria were the dominant forms of life. The next pivot brought another layer of complexity and the most consequential ratchet for the diversity of life. Photosynthesis using water, carbon from the air, and the sun’s energy meant that life could prosper anywhere with sunshine and a littlemoisture. Stromatolites, fossils formed by mats of blue-green algae that changed the world billions of years ago, can still be seen today off the western coast of Australia.
    From the perspective of the organisms that were used to living in the low-oxygen atmosphere of the time, the evolution ofphotosynthesis was a disaster. Oxygen, a by-product of photosynthesis, spiked upward in what geologists variously call the Great Oxygen Event, the Oxygen Catastrophe, the Oxygen Crisis, or the Oxygen Revolution. To organisms accustomed to a no-oxygen atmosphere, the oxygen was toxic. As that hatchet fell, those organisms retreated to stagnant waters and other airless environments. But the crisis made possible other forms of life. With oxygen in the atmosphere, microbial life could eventually evolve to complex plant and animal forms that relied on breathing oxygen to convert food to energy. The oxygen in the atmosphere also gave rise to the ozone layer, the thin shield that protects against the sun’s harmful,cancer-causing ultraviolet rays.
    Sometime around 1.5 billion years ago, more pivot points occurred. Organisms could reproduce sexually. Species could evolve more rapidly to change their shape and size and adapt to their environment based on inherited traits. Plant species evolved based on the precedent of obtaining energy through photosynthesis. An abundance of sponges, jellyfish, corals, and flatworms burst onto the scene about a half billion years later, followed byfish, insects, birds, and mammals. Here was a new life strategy. Animals could obtain energy not directly from the sun but through digesting plants and other animals in their guts. Fungi, such as yeasts, molds, and mushrooms, wound up with a strategy similar to animals. They, too, absorb their food from other plants and animals, but through their cell walls, without the benefit of stomachs.
    New species formed as continents spread apart by plate tectonics, mountain ranges, oceans, and other geographic barriers separated groups of individuals, who exchanged genes with one another andover time became distinct species. Or individuals mated with others nearby to exploita new niche or a new source of food. But the path to the diversity of plant and animal life was not smooth. The geologic record shows devastating, global-scale extinctions of plant and animal species that punctuate the long-term climb to increased diversity. At least five times these life-extinguishing hatchets fell. The most