"Researchers from North American, European and African institutions analyzed ancient DNA from 41 human skeletons curated in the National Museums of Kenya and Tanzania, and the Livingstone Museum in Zambia."

Previous archaeological research shows that the Great Rift Valley of Kenya and Tanzania was a key site for the transition from foraging to herding. Herders of livestock first appeared in northern Kenya around 5000 years ago, associated with elaborate monumental cemeteries, and then spread south into the Rift Valley, where Pastoral Neolithic cultures developed.
The new genetic results reveal that this spread of herding into Kenya and Tanzania involved groups with ancestry derived from northeast Africa, who appeared in East Africa and mixed with local foragers there between about 4500-3500 years ago. Previously, the origins and timing of these population shifts were unclear, and some archaeologists hypothesized that domestic animals spread through exchange networks, rather than by movement of people.
After around 3500 years ago, herders and foragers became genetically isolated in East Africa, even though they continued to live side by side. Archaeologists have hypothesized substantial interaction among foraging and herding groups, but the new results reveal that there were strong and persistent social barriers that lasted long after the initial encounters.
Another major genetic shift occurred during the Iron Age around 1200 years ago, with movement into the region of additional peoples from both northeastern and western Africa. These groups contributed to ancient ancestry profiles similar to those of many East Africans today. This genetic shift parallels two major cultural changes: farming and iron-working.
The study provided insight into the history of East Africa as an independent center of evolution of lactase persistence, which enables people to digest milk into adulthood. This genetic adaptation is found in high proportions among Kenyan and Tanzanian herders today."

It's behind a paywall, but since it's a Reich Lab Harvard paper, they provide access at their website.

See: Mary Prendergast et al

"How food production first entered eastern Africa ~5000 years ago and the extent to which people movedwith livestock is unclear. We present genome-wide data from 41 individuals associated with Later StoneAge, Pastoral Neolithic (PN), and Iron Age contexts in what are now Kenya and Tanzania to examine thegenetic impacts of the spreads of herding and farming. Our results support a multi-phase model in whichadmixture between northeastern African-related peoples and eastern African foragers formed multiplepastoralist groups, including a genetically homogeneous PN cluster. Additional admixture withnortheastern and western African-related groups occurred by the Iron Age. These findings support severalmovements of food producers, while rejecting models of minimal admixture with foragers and of geneticdifferentiation between makers of distinct PN artifacts."