In 1962, the ecologist Robert Whitaker set out to categorize the different realms of life on Earth. Some were deserts, others tundra, still others tropical forests. He coined a word for these inhabited environments, one that scientists have used ever since: biomes.

The planet’s biomes emerged over hundreds of millions of years. Coastal wetlands sprang up along the edges of continents about 400 million years ago. About 20 million years ago, grasslands became widespread. But the biome that we’re most familiar with — one that has a huge impact on our everyday life — is the youngest of all: the indoor biome.

When humans began building shelters about 20,000 years ago, we unrolled a welcome mat for other species. Over the past few thousand years, the indoor biome has grown to colossal proportions as cities and suburbs spread across the continents. More recently elevators and other technology have lifted the indoor biome into the sky.

If you add up the area of the indoor biome in Manhattan — including its walk-ups and high-rise apartments — it’s three times bigger than the area of the island of Manhattan itself.