Experimenting possible sentences popping out:
Life began in the sea, and probably *going to end on the land.
There is no way that gerund will work there. Looking at the sentence, a future structure is needed. A very common form of forming such structure is using be + going + to. This is fixed. You can not omit anything from it. 1
Life began in the sea, and *will probably going to end on the land.
Answering this with a quote from EnglishPage:
Simple Future has two different forms in English: "will" and "be going to." Although the two forms can sometimes be used interchangeably, they often express two very different meanings.
So, sometimes these forms are interchangeable. That means they are not used together. "Will" + "going to" isn't gonna work. The correct form of the sentence goes as following:
Life began in the sea, and is probably going to end on the land.
Regarding your last sentence:
Life began in the sea, and will probably end on the land.
Referring back to the very page:
USE 4: "Will" or "Be Going to" to Express a Prediction
Both "will" and "be going to" can express the idea of a general prediction about the future. Predictions are guesses about what might happen in the future. In "prediction" sentences, the subject usually has little control over the future and therefore USES 1-3 do not apply. In the following examples, there is no difference in meaning.
And that means the last sentence is equally valid.
1: I have seen many learners fall in the trap of omitting one of either be or to in this simple future structure, and I'm not an analyst or teacher or whatever! At least, that kind of error is frequent here.