6.16 Evidence for evolution: fossils and resistant bacteria

Fossils

Fossils may be formed:

• from parts of organisms that have not decayed because one or more of the conditions needed for decay were absent (for example, organisms trapped in amber, ice or peat bogs)
• when parts of the organism are replaced by minerals as they decay
• as traces of organisms, such as footprints, burrows and root pathways, preserved in rock.

By studying fossils in rock layers, scientists can see how much or how little organisms have changed as life developed on Earth. Older rocks lie deeper, so deeper fossils are usually older.

Why the record is incomplete

Scientists cannot be certain about how life began, because:

• many early forms of life were soft-bodied, so they left few traces
• many traces that were formed have been destroyed by geological activity, such as earthquakes and the movement of rock
• fossils only form in rare conditions, and many have not been found yet.

Resistant bacteria: evolution happening now

Bacteria reproduce very quickly, so we can see natural selection in action.

1. A random mutation makes some bacteria resistant to an antibiotic.
2. When the antibiotic is used, the non-resistant bacteria die, but the resistant ones survive.
3. The resistant bacteria reproduce and pass on the resistance allele, so the population becomes resistant.

MRSA is resistant to many antibiotics and spreads in hospitals. To slow the development of resistant strains, doctors should not prescribe antibiotics for viral or non-serious infections, patients should complete their course, and antibiotic use in farming should be restricted.