Let's imagine a very simple organism (like a single-celled bacteria) and how its defense might evolve over time: --- Step 1: Early bacteria only have a basic outer wall to protect them from some threats. This is their 'proto-immune system'. --- Step 2: A new type of virus appears that can break through the wall. Many bacteria die. --- Step 3: A few bacteria develop a slightly thicker or different type of wall by chance (mutation). These survive and reproduce. --- Step 4: Over many generations, the bacteria with the better wall become more common. This is natural selection at work. --- Step 5: Later, some bacteria might develop an internal chemical defense to neutralize viruses that get past the wall. This adds another layer of protection. --- Step 6: Over time, these simple defenses become more complex, leading to specialized cells and molecules that recognize and fight specific threats. This gradual change over millions of years is the evolution of their immune system. --- Answer: The bacteria's defense system became more complex and effective against new threats through mutations and natural selection.