The origin and evolution of life on Earth is a complex and fascinating topic that is still being actively researched. While our understanding of this subject is continually advancing, there are certain aspects that can be addressed based on current scientific knowledge.
The question of why another distinct form of life did not emerge from a "first cell" over the course of approximately 4 billion years is an area of ongoing scientific investigation. One possible explanation is that the last universal common ancestor (LUCA), which is the most recent organism from which all currently existing life on Earth is descended, possessed traits or mechanisms that prevented the emergence of a separate lineage. The LUCA may have represented a successful evolutionary solution that outcompeted or integrated potential alternative forms.
It is important to note that the precise nature of the LUCA and the mechanisms by which it gave rise to diverse life forms are still the subject of scientific inquiry. It is possible that the LUCA itself was already a complex and diverse organism, and subsequent evolutionary processes led to the branching and diversification of life as we know it today.
Regarding the traceability of all known life to a single origin, the concept of a universal common ancestor is supported by a substantial body of evidence. Comparative genomics, the study of DNA and genetic similarities across different organisms, provides strong support for a shared ancestry. There are remarkable genetic and biochemical similarities among all forms of life, including fundamental cellular processes and genetic codes.
The Tree of Life, which represents the evolutionary relationships among organisms, shows a branching pattern that suggests common ancestry. However, it is important to note that reconstructing the precise details of evolutionary history, especially events that occurred billions of years ago, presents significant challenges. The study of ancient DNA, fossil records, and comparative genomics provides valuable insights but may not provide a complete and definitive picture.
As our understanding of life's origins and evolution continues to advance, ongoing research and discoveries will refine our knowledge and provide further insights into the complex processes that led to the diversity of life on Earth.