Which of the following occurred last? Is it (A) the formation of the Milky Way, (B) the formation of the solar system, (C)
the development of life on Earth, or (D) the big bang?
Let’s start by recalling that the big bang was the first event in the universe. The occurrence of the big bang led to the formation of everything else that exists in
the universe now. So we know that the big bang occurred first, not last. We should eliminate option (D).
Looking at the remaining answer choices, we might notice a pattern. That is, the Milky Way contains the solar system, and the solar system contains
Earth. The formation of all of these depend on each other sequentially. Earth could not have formed without the solar system, and the solar system could not
have formed without the Milky Way. This is a good hint that the development of life on Earth, option (C), must have
occurred last. To be sure though, let’s take a closer look at all of the remaining options.
Option (A) says the formation of the Milky Way. Interestingly, the Milky Way formed only a few hundred million years after the big
bang, making it quite old compared to many galaxies. Now, we recall that a galaxy is a collection of billions of stars, but this doesn’t
mean that all the stars in a galaxy form at the same time. In reality, a galaxy can go through different phases of star formation throughout its
existence. Perhaps surprisingly though, the Milky Way was almost 10 billion years old by the
time that the Sun formed. So, because the formation of the Sun, and later the rest of the solar system,
occurred billions of years after the formation of the Milky Way, we can eliminate
This leaves us with options (B) and (C). So, to answer this question, we need to determine whether the formation of the solar
system or the development of life on Earth occurred last. We might recall that the solar system formed about four billion years ago. The Sun formed first followed by the planets, including Earth. Then, once Earth was formed, it was another several hundred million years before life
developed on the planet in any form. Thus, we know that life on Earth developed after the solar system formed, so we can
eliminate option (B). This confirms that the correct answer is (C). Of the events listed here, the development of life on Earth occurred last.