In July 2002, scientists announced the discovery of the

I did not notice "to walk on" and picked (E)

+ 1 D

That's an easy one.

In July 2002, scientists announced the discovery of the first living creature on Earth to walk on four legs and which was 350 million years old.
A) In July 2002, scientists announced the discovery of the first living creature on Earth to walk on four legs and which was 350 million years old.
=> This means that scientists announced the discovery to walk on four legs. Their main aim was to walk on four legs.
B) Scientists announced the discovery on Earth of the first living creature which walked on four legs and which was 350 million years old in July 2002.
=> The use of which is not correct.
C) Scientists announced the discovery on Earth of the first living creature in July 2002 which walked on four legs and which was 350 million years old.
=> The use of which is not correct.
D) In July 2002, scientists announced the discovery of the first living creature on Earth which walked on four legs and which was 350 million years old.
===> This choice is not correct but best of the worst options. The use of which is not correct. Before which we should always use comma. All OG's (OG10, 11, 12) say that "which" should be used as non-restrictive. That means we should use comma before which.
E) Scientists announced the discovery of the first 350 million-year-old living creature on Earth to walk on four legs in July 2002.
=> The problem is in "to walk".

Some thoughts:
1. Pay attention on the meaning of the sentence:
Example:
I discovered some techniques to solve the problems.
=> Why did I discover? To solve the problems.
2. Whenever we use "which" we should use comma before "which", which should be used as non-restrictive clause.
Example:
I live in a Delhi, which is the capital of India.
=> I live in a city that is the capital of India.

If comma is not used before "which" in option D, none of the options is right.

Comments are welcome.