Hey there! I'm happy to help!
The Earth is extremely ancient (approx. 4.6 billion years), so its milestones are going to have to be something huge and very grand in scope that has also happened a few times.
Changes in climate can be significant, but these are not as important of an event in Earth's history to mark the different eras. While changes in climate can lead to more drastic things, the changes on their own are not good enough of a definer.
Asteroid impacts are pretty huge, but it doesn't make sense to have each era be marked by an asteroid impact because it is an extremely specific type of event and it hasn't happened a large amount of times in Earth's history, so this isn't going to be the principal marker.
Old scientists in lab coats is just a play on words of the question. Obviously the paleontologists and geologists chose what to have define the different eras, but it isn't like the scientists themselves ARE the actual markers.
The eras of the geologic time scale are best defined by C. mass extinction events. Asteroid impacts were under the umbrella of this, and that is why it wasn't the right answer. This is a much more broad term that represents the times that Earth was completely changed whether that be from an asteroid, an ice age, a volcano eruption, etc. These are huge in scope and happened many times in the history of the Earth, so it makes to have these mark the different eras.
Have a wonderful day and keep on learning! :D
