Paleontologist: It is widely, but falsely, held that life began in the ocean and did not exist on land until half a billion years ago.
Conclusion
It's false to say that life began in the ocean and didn't exist on land until half a billion years ago.
Evidence
We've found carbon 14 on 1.2 billion year old rock in North America, and carbon 14 is extracted by plants and microbes from the atmosphere and released when they die (i.e. it's can be a marker of life).
Evaluate
Since we're doing Strengthen EXCEPT, we'll need to find four ways to strengthen the argument. Often, a strengthen answer is really just ruling out a weakening objection, so we're wise to think through some possible objections.
If you were trying to defend the widely held belief that life on land didn't exist until half a billion years ago, how could you explain this carbon 14 on 1.2 billion year old rock?
Maybe we'd say that at the time of its getting smeared with carbon 14, that 1.2 billion year old rock was actually underwater.
Maybe there are other explanations for how carbon 14 gets on a surface (the fact that living things use it and release it doesn't convince us that this is the only reason you might ever find carbon 14 somewhere).
Goal
Look for answers that rule out possible objections or that add some more evidence that life on land may have existed prior to 500 million years ago.