This would have been at a period before any real sophisticated attackers exist, and before life had built up any specific requirements of an "internal" environm

This would have been at a period before any real sophisticated attackers exist, and before life had built up any specific requirements of an "internal" environment too. It seems intuitive that this would be how it started, goopy little bubbles filled with goo, just barely keeping out the other goo that would disrupt it.