The subsurface ‘deep biosphere’ represents one-tenth to one-third of Earth's total global present-day biomass. The rest is dominated by land plants, a relatively recent development in geological history. Before c. 400 Ma, a relatively low surface biomass with high productivity and fast turnover supplied carbon to a deep biosphere with high biomass but low productivity and slow turnover. Here, we argue that the deep biosphere outweighed the surface biosphere by about one order of magnitude for at least half of the history of life on Earth. This result offers a new perspective on the history of life on Earth with important implications for the search for life on other worlds.