In the 19th and early 20th centuries, geologists hypothesized that major landmasses were once connected via an elaborate series of land bridges. This was an attempt to explain the distribution of plants and animals around the world, as it was recognized that populations could not have radiated across the world as they had with the continents in their present configuration.
The land bridge theory explained how life could have populated different continents by imagining an ever-increasing number of now-submerged land bridges. Though land bridges have existed in some places, such as the Bering land bridge that allowed humans to populate the Americas, the high number and irrational behavior of the proposed land bridges rendered the theory unworkable. By the early 20th century, one land bridge was supposed to have connected Brazil with Africa, which explained the similarities among African and South American rodents, and another bridge was supposed to have connected Europe with North America. Another bridge, or perhaps a lost continent called "Lemuria," was believed to have spanned the Indian Ocean. None of these land bridges and lost continents left any geological trace of their presence, and the theory was eventually scrapped with the acceptance of plate tectonics as a mechanism for continental drift.