Final answer:
Life on Earth likely evolved from a single origin because of the shared genetic and biological characteristics across all forms of life, which suggest a common ancestry rather than multiple, independent origins.
Step-by-step explanation:
It is highly unlikely that the existing branches of life evolved from multiple origins because of the unity of life that links all species back to a single ancestral lineage. This connection is supported by shared cellular structures, nucleic acids, proteins, and metabolic pathways that indicate a common ancestry with all life. The "Tree of Life" constructs, based on genetic analysis and RNA sequences shared across species, further support the idea that all organisms diverge from a common root, suggesting a single origin of life that has since branched out and evolved into the vast diversity we see today.
Additionally, if multiple origins of life had occurred, we would expect to see a more disjointed and less unified pattern in the genetic makeup of current organisms. However, the evidence, such as the presence of a biological clock across different species and the broad applicability of the endosymbiont theory, implies that life forms continued to evolve from an ancestral cell through natural selection and other evolutionary processes. Furthermore, notable evolutionary events like the appearance of eukaryotes through endosymbiosis suggest merging events within the tree of life rather than completely separate lineages arising independently.