Self-paced
Support Even the earliest known species of land animals, known from fossils dating from the late Silurian period, 400 million years ago, show highly evolved adaptations to life on land. █████ ███████ ███████ ███ ██████████ ███████ ███████ █████ ████████████ █████ ███████ ██ ████ ███████ ████ ████ ███████ ████ ███████ █████ ███████ ██ ███████ ████████████
Argument Breakdown
The argument concludes that early land animals must have evolved rapidly when they first emerged from aquatic environments. How do we know? Because the earliest known land animals were highly adapted to living on land. But these adaptations aren't present in aquatic animals or amphibians. So, we can infer that the earliest known land animals must have evolved these adaptations.
Objective: Identify a Necessary Assumption
If we look closely, we can see a gap between the inference we can draw from the premises, and the argument's actual conclusion: timing. The argument lets us be pretty confident that certain adaptations were evolved between land animals first leaving the sea, and the emergence of the earliest known land animals. But how early were the earliest known land animals, actually?
To conclude that these adaptations were evolved rapidly after leaving the water, we need to assume that the earliest known land animals had emerged from the water fairly recently. In other words, that land animals first emerged not long before the late Silurian period. Otherwise, there could have been a super-slow adaptation process in a missing piece of the fossil record.
Of course, it's always possible that the correct answer will identify another necessary assumption that we overlooked. Process of elimination is always a safe bet, using the negation or must-be-true tests to figure out whether each answer choice is truly necessary.
21.
Which one of the following ██ ██ ██████████ ██ █████ ███ ████████ ████████
a
Known fossils of █████ ████ ███████ ███████ ███████ ██ ███████ ████ █████ ██████████ ████ █████ ███ █████ █████████ ██ ████ ████████
In other words, the earliest known land animals must include animals that had only recently emerged from water. And if those animals already had significant adaptations, it allows the argument to make sense in saying that the adaptations evolved rapidly.
If we negate (A), we can confirm its necessity. If known fossils don't include early land animals, that means we don't know how fast the adaptation process was. Without a truly early fossil record, the adaptations could have evolved quickly or slowly. So the argument falls apart.
b
Fossils from the ████ ████████ ██████ █████████ ████ █ █████ ██████ ██ ███ ██████ ███████ ████ ████ █████ ██ ████ █████
If anything, narrowing the range of species represented by fossils would make the argument weaker: it means there's more we don't know. If (B) were negated, and a large number of species were represented by fossils, that wouldn't harm the argument at all.
c
No plants were ███████████ ██ ████ ██████ ███ ████ ████████ ███████
The argument is just about animals, so as far as we know, plants are beside the point. If we negate (C) and say that there were land plants before the late Silurian period, it doesn't affect the conclusion one way or another.
d
No present-day species ██ ███████ ██████ ██ █████████ ████ █ ███████ ██ ██████ ████ ████ █████ ██ █████
Aquatic (and amphibious) animals are only mentioned in the argument to show that the adaptations weren't already present when land animals first emerged. But since the argument is just about those early land animals, what happened afterwards doesn't make much difference.
We can see that (D) isn't necessary by negating it. If we say that some present aquatic species are descended from land animals (as with whales, for example), that doesn't harm the argument. It doesn't prevent the possibility that early land animals rapidly evolved to adapt to the land.
e
All animals alive ██ ███ ████ ████████ ██████ █████ ██████ ███████████ ██ ████ ██ ███████████ ██ ███ ██████
The argument makes a distinction between aquatic, amphibious, and land animals—which suggests that at least in general, some animals do live exclusively on land or in water. But there's no need for these categories to have been exclusive in the late Silurian period.
We can see this by negating (E). If there were amphibians present at the time, that doesn't harm the argument. We know that early land animals