Darwin's theory of evolution, as outlined in his seminal work "On the Origin of Species" (1859), explains the process by which species evolve and diversify over time through natural selection. It does not claim that humans evolved from modern-day species, but rather proposes that all living organisms, including humans, share a common ancestry and have evolved from simpler life forms over millions of years.
The key concept in Darwin's theory is "descent with modification." This means that species change over time, and new species arise from pre-existing ones through a gradual process of adaptation to their environment. The process of evolution occurs through several mechanisms, including natural selection, genetic variation, and reproductive isolation.
The process of human evolution, as supported by extensive fossil and genetic evidence, suggests that our species, Homo sapiens, shares a common ancestor with other great apes, such as chimpanzees and bonobos, which lived several million years ago. This common ancestor was not Homo sapiens but a different species that gave rise to both humans and modern apes.
Over time, as this common ancestor population became geographically isolated or encountered different environmental conditions, variations in traits (genetic mutations) accumulated in different populations. These variations resulted in some individuals having advantageous traits that allowed them to survive and reproduce better in their specific environments. This process of natural selection favored the survival of those individuals with beneficial traits, leading to the gradual evolution of new species, including our ancestors.
Human evolution is a complex and ongoing process that involves multiple species and transitional stages. The fossil record, combined with genetic evidence, provides a detailed picture of the gradual changes that have occurred over millions of years, ultimately leading to the emergence of modern humans.
In summary, Darwin's theory of evolution does not propose that humans evolved from modern-day species directly. Instead, it explains how all living organisms, including humans, share a common ancestry and have evolved from simpler life forms through a process of descent with modification and natural selection.