Question

question 3 1 pts
which line of evidence best supported researchers claim that cell-like structures in apex chert were actually fossilized cells?
○ their presence in oxygen-rich rock layers
○ the large size of the microstructures
○ their association with modern cyanobacteria
○ carbon isotopic signatures consistent with biological activity (fractionation)

Explanation:

Biological organisms preferentially use lighter carbon isotopes ($^{12}$C) over heavier ones ($^{13}$C), creating a distinct isotopic signature (fractionation) that differs from non-biological carbon. This signature is a key, direct marker of past biological activity, making it the strongest evidence for fossilized cells. The other options are not reliable: early life existed in low-oxygen environments, large size does not indicate biology, and association with modern cyanobacteria alone is not definitive proof of fossilized cells.

Answer:

Carbon isotopic signatures consistent with biological activity (fractionation)