How do scientists know about how many species have existed in the past?

Scientists estimate the number of species that have existed in the past through the study of the fossil record, which provides evidence of extinct organisms and their diversity over time. They also use techniques such as paleobiology and comparative anatomy to analyze morphological traits and evolutionary relationships. Additionally, statistical models and biodiversity estimates help extrapolate the total number of species, taking into account known species and their ecological roles. However, many species remain undiscovered, so these estimates are inherently incomplete.