The fossil record

The paleontological criterion of character polarity is as follows:

The ancestral state of a character must have preceded the derived states and therefore the earlier state in the fossil record is likely to be ancestral.

For example, in the evolution of the birds from reptiles, some characters changed: posture evolved from a sprawling to an upright gait. Some of these characters leave a fossil record and we can infer which character states were ancestral and which derived by seeing which is found in the earlier fossils. The image opposite for example is a famously well preserved archaeopteryx, ancestor to modern birds.

However, if the fossil record is less complete then a derived character could be preserved earlier than its ancestral state. With a highly incomplete record, the evidence may be practically worthless.