* Early Earth Conditions: The early Earth was a harsh environment, with no oxygen in the atmosphere and extreme temperatures. Prokaryotes evolved to thrive in these conditions, while eukaryotes, with their more complex structures, likely couldn't have survived.

* Simpler Structure: Prokaryotes are simpler organisms, lacking the complex internal structures (like a nucleus and membrane-bound organelles) found in eukaryotes. This simpler structure allowed them to evolve and reproduce more quickly, adapting to the changing conditions of early Earth.

* Fossil Evidence: The oldest fossil evidence of life on Earth is of prokaryotes, dating back billions of years. The first eukaryotes appear much later in the fossil record.

* Endosymbiotic Theory: The prevailing theory is that eukaryotes evolved from prokaryotes through a process called endosymbiosis. This involves a larger prokaryote engulfing a smaller one, which eventually became an organelle (like mitochondria or chloroplasts) within the larger cell. This suggests that prokaryotes were already present before the evolution of eukaryotes.

In essence, prokaryotes were the pioneers of life on Earth, adapting and evolving to survive in a challenging environment. Eukaryotes, with their more complex structures, appeared later, likely evolving from prokaryotes themselves.