1. Fossils can show the gradual change in structures over time that link organisms that seem very different – for example, fish and reptiles. Transitional fossils such as Tiktaalik match predictions for organisms that are adapting to habitats that are less aquatic than where their ancestors thrived. Also, similarities in the structures of different animals (ex. similar forelimbs in bats, birds and humans) provide evidence for common ancestry.
2. Eusthenoperon was a fish that lived about 400 million years ago. It clearly did not leg-like limbs. Some of its descendants such as Tiktaalik had fins with bones that would have allowed it to survive in shallower waters. Acanthostega had lungs and limbs that could still paddle in water but not walk on land. Descendants with stronger leg bones were eventually able to thrive both in and out of the water in shallow aquatic areas. These descendants have been classified Ichthyostega.
3. Natural selection chose for favorable traits such as a flatter head, paddle-like legs and eventually lungs (in addition to gills) that would have made living in shallow waters easier. In shallow waters, species such as Tiktaalik and Acanthostega would have faced fewer predators and more food since fish adapted to deeper waters could not followed. Those fish with slightly flatter heads or slightly thicker bones in their fins would have had an advantage in shallow waters. This means they would have survived and had more offspring that would inherit these helpful traits.
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