Q4. Which reaction might the first oxygen-releasing cells have used, according to NCERT?
One similar to the light reaction of photosynthesis, in which water is split with solar energy captured and channelised by light-harvesting pigments (NCERT pages 121–122).
Q5. What happened to the mammals of South America when South America joined North America?
They were overridden by North American fauna (NCERT page 124). Beyond the NCERT text: the land connection enabled a two-way interchange, with unequal migrations and extinctions, not a complete replacement.
Q6. In NCERT Figure 6.10, into which two lines do the early reptiles split, and which one leads to the mammals?
The sauropsids and the synapsids; the synapsids, through the pelycosaurs and therapsids, lead to the mammals (NCERT Figure 6.10, page 123).
Q7. Name the three eras of NCERT Figure 6.9, oldest first, with one period of each.
Paleozoic (for example Devonian), Mesozoic (for example Jurassic) and Cenozoic (for example Tertiary) (NCERT Figure 6.9, page 122).
Q8. Which of whales, dolphins, seals and sea cows are fully aquatic, and which are semiaquatic?
Beyond the NCERT text: whales, dolphins and sea cows are fully aquatic; seals come onto land or ice to rest and breed, so they are semiaquatic (Wikipedia, “Pinniped”, facts in our words). NCERT page 124 lists all four among the mammals that live wholly in water; in an NCERT-based answer, give NCERT’s four examples.