The Jebus Said So.

There's a number of ways to sort out what predated Mark or was in an earlier redactional layer and what's in the latest layer of Mark.

One way would be looking at the fringe details that are at odds with canonical church theology and organization at the time it is written.

For example, "carry no purse" in Mark 6:8 would inhibit the ability to make monetary collections. It is very problematic for Paul, and you can even see in 1 Cor 9 that the Christian community in Corinth who he later accused of accepting a different version of Jesus and later depose Rome's appointees have attitudes against Paul's doing so which he's arguing against.

This makes its way via copying into both Matthew and Luke, but then in a post-Marcion version of Luke a line is added at the last supper explicitly reversing not carrying a purse.

Another is Mark 11:16, where he doesn't allow anyone to carry sacrifices through the temple. This isn't found in Luke or John, and Matthew who is copying it verbatim explicitly left out this part. It's a problem because what was carried through the temple were sin sacrifices, and Jesus hasn't died yet. Cannonical theology is that it was his death that wiped the record clean. So while the other gospels have no issue with his criticism of selling the sacrifices in front of the temple, prohibiting any sacrifices at all from being carried through the temple is a big problem.

Here too this seems more in line with the attitude in Corinth Paul was arguing against that "everything is permissible for me" - which it's worth noting he doesn't outright reject but instead tried to make a relativist argument around in interpretation.

Another way we can evaluate Mark is looking at competing textual traditions.

So in the first case above we see a similar but different statement in the middle of Thomas 14 which only endorses accepting food and shelter, and a number of unique sayings in agreement with the concept of not paying for religious service such as sayings 88, 21, and 109.

And again, we can see the similar attitudes pre-existing Paul's first letter in 1 Cor 9.

For this to be originating in Mark sometime after 70 CE seems highly unusual as it opposes Paul's fundraising efforts, mirrors earlier attitudes in Corinth, and reflects positions in sayings unique and similar to it in an extra-canonical text.

In terms of the second, an identical pattern emerges.

In Corinth Paul is arguing against the position that "everything is permissible," then later on in Mark is a line effectively prohibiting sin sacrifices, and in Thomas we see a similar attitude rejecting propiation and instead furthering a relative picture of morality in sayings 6 and 14.

As a bonus, saying 14 doesn't just cover no propiation and not carrying a purse, both mirroring ideas Paul is arguing against in Corinth, it also covers absolutely eradicating eating restrictions which Paul also argued against with Corinth without directly opposing it and instead appealing to a counter-interpretation based on relative mortality.

So you end up with this picture where Paul is arguing against something, Mark contains some offhand mention supporting the thing Paul was arguing against, later cannonical texts attempt to reverse that, and Thomas contains sayings similar but not identical to canonical sayings supporting those things in entirely different contexts as well as unique sayings that support the same positions.

This is exactly the pattern we should expect if Paul and the canonical tradition was actively working to undermine an earlier and separate tradition of teachings attributed to Jesus.

And here too, the concepts Paul and the later texts are arguing against happen to be in line with the Greek philosophy of Epicurus as recorded in Lucretius who complained that people were too caught up in worrying about what gods thought and there was no point in sacrificing, giving money to religion, or other religious obligations. A philosophy that has its other elements heavily engaged with across multiple sayings in Thomas and attributed with the opposition to the physical resurrection Paul is arguing against in 1 Cor 15.

Which then brings us back to things like Mark 4 where a saying about randomly scattered seeds where only what survives multiplied which even uses the specific phrase of "seed falling by the wayside of a path" - just like Leucretius used decades earlier to describe failed reproduction - is being depicted as being said in public. Followed by a private explanation which is one of only two explanations in antiquity about this parable, with the other being provided by the Thomasine tradition interpreting it in line with Lucretius's depictions of seeds as atomos and also using language identical to Lucretius's in doing so. A parable which appears in Thomas without the secret explanation of Mark and the other Synoptics.

I very much doubt Mark was inventing a parable paraphrasing Lucretius, then having it said in public but secretly explained with an interpretation at odds with Lucretius in private, and then a later tradition well after Lucretius was no longer popular reinterpreted it back to Lucretius using his language again in doing so, and that tradition just so happened to also have variations of several sayings in Mark which were problems for the earlier church and in agreement with opposing views in Corinth but in line with Lucretius's perspectives, as well as multiple unique sayings with similar attitudes. (Along with multiple sayings and ideas Paul seems to be quoting in his letter to Corinth.)

This is all much more easily explained by a pre-Pauline tradition that was influenced by and engaged with Epicurean philosophy and Lucretius specifically which was problematic for Paul's associations in Jerusalem which he and what later becomes the canonical church attempt to spin back towards conservative Judaism with increasing success over the next century, with Mark as one of the earliest attempts to do so in narrative form. But as the first attempt, it did so sloppily enough small parts of the opposed positions peek through before being better corrected by later efforts.