Nothing in science is ever 100%. All we have are fossils and artifacts from that period. We can't go back in time and examine the brain structure of all the species of hominids. The only 3 facts we know for certain are:
1) human skulls have not changed for about 200,000 years
2) there was no improvement in stone tools or symbolic art from about 200,000 to 60,000 years ago
3) there was a near instant increase in our making of tools/weapons/art within a 10,000 year period and we proceeded to wipe out all other hominid species still around
This fits with the idea that it was a single mutation that rewired the human brain, resulting from some biological law, that produced the Merge function. Infinitely recursive systems are extremely rare in nature, you need a very complex system like the human brain for it to form.