Here's why:
* Early Earth was a Gene-Sharing Soup: In the early days of life on Earth, the environment was vastly different. Organisms were simpler, and cell membranes were more permeable. This allowed for the free exchange of genetic material between different lineages, even across domains.
* Blurring the Lines of Ancestry: HGT is the transfer of genetic material between organisms that are not directly related through descent. This process blurs the lines of traditional ancestry, making it difficult to trace the evolutionary history of a gene or organism.
* Complicated Evolutionary Histories: HGT introduces complex evolutionary relationships, where a gene in one organism might have originated from a completely different lineage. This makes it difficult to reconstruct the tree of life based solely on gene sequences.
To summarize: While vertical gene transfer (from parent to offspring) is the primary mode of inheritance, HGT was a significant factor in the early evolution of life on Earth. It significantly complicates efforts to understand the evolutionary relationships between the three domains (Bacteria, Archaea, and Eukarya).
