On 2015 Oct 13, Francesco Buonocore commented:

As you can see from: Bioessays. 2015 Aug;37(8):877-87. "Evolution of vertebrate adaptive immunity: immune cells and tissues, and AID/APOBEC cytidine deaminases." from Hirano M., one of the theory is that: "Around 500 million years ago, a common ancestor of jawed and jawless vertebrates evolved a genetic program for the development of prototypic lymphoid cells as a foundation for an adaptive immune system. This acquisition preceded the convergent evolution of alternative types of clonally diverse receptors for antigens in all vertebrates"

On 2015 Oct 02, Jens Staal commented:

Cool! Personally I am very curious about the adaptive immune system development between the lampreys and the sharks - especially since homologs of my "pet protein" MALT1 underwent a double duplication event from PCASP3 in lampreys to PCASP1(MALT1), PCASP2 and PCASP3 in sharks (and the rest of the vertebrates except mammals). My "gut feeling" says that this has something to do with the evolution of the adaptive immune system found in jawed vertebrates, since MALT1 is a critical signaling protein in T- and B-cell antigen receptor signaling.

On 2015 Oct 02, Jens Staal commented:

Cool! Personally I am very curious about the adaptive immune system development between the lampreys and the sharks - especially since homologs of my "pet protein" MALT1 underwent a double duplication event from PCASP3 in lampreys to PCASP1(MALT1), PCASP2 and PCASP3 in sharks (and the rest of the vertebrates except mammals). My "gut feeling" says that this has something to do with the evolution of the adaptive immune system found in jawed vertebrates, since MALT1 is a critical signaling protein in T- and B-cell antigen receptor signaling.

On 2015 Oct 13, Francesco Buonocore commented:

As you can see from: Bioessays. 2015 Aug;37(8):877-87. "Evolution of vertebrate adaptive immunity: immune cells and tissues, and AID/APOBEC cytidine deaminases." from Hirano M., one of the theory is that: "Around 500 million years ago, a common ancestor of jawed and jawless vertebrates evolved a genetic program for the development of prototypic lymphoid cells as a foundation for an adaptive immune system. This acquisition preceded the convergent evolution of alternative types of clonally diverse receptors for antigens in all vertebrates"