1. Chapter 18 Life in the Universe

2. Chapter 18 Life in the Universe
The only place we know life exists is here on Earth
One of humanity’s Big Questions is whether it exists elsewhere
We can get some clues by considering life’s history here on Earth
When we do, we can get an idea how likely life “as we know it” is
But we should keep in mind that life as we know it may not be the only possible kind of life
It is, however, the kind that we will be best able to recognize, if it does exist…

3. When did life arise on Earth?

4. When did life arise on Earth?
The first evidence of life appears ~150 million years after the end of the Era of Heavy Bombardment
This is not fossil evidence, but trace chemical evidence
The evidence is in the ratio of carbon- 12 to carbon-13

5. Carbon isotope evidence for life
In >3.8-billion-year-old rocks like these in Greenland there is a higher than normal ratio of 12C:13C
Living things incorporate 12C more easily than 13C
So the higher ratio is taken as indirect evidence for life