Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A geologist found fossils of horse bones in different rock layers of a canyon. The fossils in the lower levels show shorter bones than those in the upper-level fossils. Which conclusion is based on the geologist's observations?
Horse's became larger over time.
The horse population evolved larger bones over time.