Once upon a time, there were very few people within a
relatively small part of the world. All of them were very similar genetically
because they shared the same small group of ancestors. If scientists could have
performed Y-DNA testing on them, they would have had nearly identical test
It should be noted that there were other humans in the world at about the same time. It's just that the other branches of the human tree did not survive --- only those from whom the present human population is descended.
Then one day, for some unknown reason, there was a single change in the Y-DNA which was passed from father to son. This is called by scientists a polymorphism, or single-nucleotide polymorphism (SNP). Such a change is considered quite rare, and a specific SNP is considered to have occurred only once in human history. After such a change, we now have two different groups --- those without the SNP, and those with the SNP. In scientific terminology, these first groups were labeled "Y" and "A" haplogroups. The first SNP recorded for this group was called M91. If you have the M91 SNP, you are in the "A" haplogroup. If you do not have it, you are still in the "Y" haplogroup.
A brief description of Haplogroups is given at http://www.ivey-ivie-ivy.org/DNA/haplogroups.htm. There is also a table from a paper by FamilyTreeDNA referenced at http://www.familytreedna.com/haplotree.html. In this table you can see at the top of the page the letter "A" and to the right of it the M91 SNP.
After many more years (remember, these SNPs are quite rare), the guys in the "A" haplogroup split into 4 groups:
A* --- have only the M91 SNP
A1 --- have the M91 SNP and a new M31 SNP
A2 --- have the M91 SNP and several new SNPs
A3 --- have the M91 SNP and a new M32 SNP
After many more years, the A3 haplogroup splits into 3 groups:
A3* --- have only the M91 and M32 SNPs
A3a --- have the M91, M32, M28, and M59 SNPs
A3b --- have the M91, M32, M144, M199, M220 SNPs
As specific examples, those of us who are in the Ivey-Ivie-Ivy surname project, have the following SNPs:
E1b1a --- SRV10831.1, M42, M94, M139, M168, P9, M145, M213, Yap, SRY4064, M96, P29, P2, DYS391p, M2, P1
E1b1b --- SRV10831.1, M42, M94, M139, M168, P9, M145, M213, Yap, SRY4064, M96, P29, P2, DYS391p, M35
R1a --- SRV10831.1, M42, M94, M139, M168, P9, P14, M89, M213, M9, M45, M74, P27, 92R7, M207, UTY-1, M173, SRY10831.1
R1b --- SRV10831.1, M42, M94, M139, M168, P9, P14, M89, M213, M9, M45, M74, P27, 92R7, M207, UTY-1, M173. P25
If you have followed all this so far, you might ask "Why does FamilyTreeDNA bother with the 12- and 25-marker tests for Haplotypes", and "What do SNPs have to do with determining family relationships?"
For the answers, see "Tutorial on Y-DNA Haplotypes".