Q19 - Paleontologist: It is widely

[mrudula_2005]

Why/how does D not provide additional support for the paleontologist's conclusion?

I didn't quite get LSAC's explanation - that D indicates that plants and microbes were not the ones to extract the Carbon 14 from the atmosphere (how do we even know that? just because it came directly from the atmosphere into the soil?), but how do we know that in C the plants weren't similarly uninvolved? i'm confused here..thanks

[ManhattanPrepLSAT2]

To support the conclusion, we want to discredit either the idea that life started in the oceans or that it didn't exist on land until 500 million years ago.

I think (A), (B), and (E) do one or the other pretty directly, and can quickly be eliminated.

(C) and (D) involve a bit more. To discuss these, let's think about the argument core.

In this argument, the author is combining two pieces of evidence to formulate a conclusion:

traces of carbon-14 have been found throughout certain 1.2 billion year old rocks

+

carbon 14 is extracted by plants and microbes from atmosphere, then released when they die

=

It is false that life began in the ocean and did not exist on land until 1/2 a billion years ago.

What is the author assuming in combining the two pieces of evidence? That the carbon-14 excreted by life forms is the carbon-14 that has been found in the rocks.

Therefore, we can strengthen the argument by showing that either--
a) this is indeed the carbon 14 in the rocks or
b) something else isn't responsible for the carbon 14 in the rocks

(C) is a challenging answer. I believe your point was that perhaps the carbon 14 could have come from water-based life forms. However, keep in mind that carbon 14 is created, per the passage, in a situation where plants and microbes react w/the atmosphere -- that is, when plants and microbes are not in water.

Therefore, (C) doesn't provide support for the idea that the carbon 14 could have been from something else.

Instead, what it does do is give us a potential way in which life could have come to land more than 500 million years ago -- maybe the land was submerged, and then became dry. If the oceans had life, it would make sense that perhaps some of that life would stay behind on dry land. Not 100% proof, but certainly support for the idea that life existed on land.

(D) would weaken the argument, because it exposes the assumption in a bad way -- perhaps the carbon 14 in the soil is not from life forms on the land.

[interestedintacos]

The frustrating thing about D is that it could still be used to support the argument, just like other new premises that have been introduced in the answer choices. But I suppose because D conflicts with a premise/assumption of the argument as stated, it's not considered a strengthener.

Would you say that's correct?

[ManhattanPrepLSAT2]

The author is assuming that the carbon 14 in the rocks is proof of life existing on land.

What (D) tells us is that that carbon 14 could have been there without any life -- The carbon 14 could have gone directly from the atmosphere into the rocks.

I think I understand how you are seeing it -- hey the plants could have gotten C14 from the atmosphere, used it, then released it back into the atmosphere, where then it got into the rocks, but I think that requires more conjecture.

[interestedintacos]

Actually what happened is after strongly considering choice C I lost track of what was actually at stake. I was too focused on the issue of whether the rocks were in the water or not. So I was thinking if the C-14 came straight from the atmosphere that means the rocks must have been outside of water, as opposed to C-14 coming from the atmosphere to water, to rocks.

But obviously what matters is whether or not we have evidence of life, not whether the rocks were in water or not. It shows how answer choices can lead you away from what's at stake in the stimulus--and it shows the necessity of checking each answer choice against the stimulus and keeping straight exactly what's at stake.

So yeah, now I see it accurately and see that choice D doesn't support the argument or conclusion at all.

[jlz1202]

I don’t understand why "(C) addresses this issue (since we know carbon 14 is found throughout all the rocks" ? The stimulus says "...found throughout certain 1.2 billion=year-old rocks" I thought that C14 would be among a portion of (not all)1.2 billion-year-old rocks.
Therefore I am confused by choice (C) : "some portions of 1.2 billion...rocks submerged in water though other portions clearly never were." I think the options not submerged in water do not necessarily overlap with the rocks containing C14 thus (C) cannot strengthen the conclusion.
Anyone helps? Really appreciate it!

[ManhattanPrepLSAT2]

Remember that we are looking for an answer that supports the conclusion, not necessarily one that has to prove it.

The fact that some of the rocks were submerged in water (where there was life), supports the idea that there was life on these rocks before 500 mil years ago. Could it be that there isn't an overlap in the rocks that were underwater and those that contain carbon? Certainly. Therefore, the argument isn't 100% perfect. However, that doesn't mean (C) doesn't support the argument. We're trying prove that there was life before 500 mil years ago, and (C) gives us a pretty strong explanation of how that could have happened.

Hope I've addressed your question and that the above is helpful -- please follow up if you want to discuss further!

[jlz1202]

Oh now I understand! Thank you so much for pointing it out! ! So (C) is trying to say: at least there are some rocks on land and so at least there is a possiblity of some rocks on land containing C14 therefore at least there is a possibility of some rocks containing C14 released by ancient plants/microbes on land.

Thus there is a possiblity that life origins from land.

Is my understanding correct?

[ManhattanPrepLSAT2]

I think we're almost in agreement --

I also realize I could have created some of the confusion with a terrible explanation above -- I've amended it --

What (C) gives us is a possible way that life could have gotten to land more than 500 million years ago -- some of that land could have been submerged in water (where we know life existed). As the water receded from the rocks, it would make sense that life could transfer from water to land. Again, not 100% provable by any means, and, even if it were, not 100% proof that this transfer happened over 500 million years ago, but by giving a means by which life could have gotten to land, it strengthens the idea that it indeed did.

[bearknowsthetrooth]

When the paleontologist states that it's falsely held that "life began in the ocean and did not exist on land until half a billion years ago" doesn't he mean that it's not true that life began in the ocean? Or is only the second part of that statement false?