Agents come up from the ranks, starting as interns at a literary agency to get a feel for the business. In time they might become an assistant to an agent and learn how it's done before moving up to the role of junior agent.
At some point, those who are eager to get ahead will move to a bigger agency. Some will leave that bigger agency and start their own firm.
Query the new agent who is looking to build a client roster. Query the newly independent agent who has a business to fill with clients.
Alex Glass has gone off on his own and he's open to queries. Not historical fiction, but if you have some literary fiction, family saga or YA, you might consider shooting off a query. While he may have brought most of his clients along with him in the move, he won't build up his business without fresh manuscripts from unknown writers. It's worth a try.
At Curtis Brown, Kerry D'Agostino has made the move from assistant to agent and she needs clients. You could be one of them if your query draws her attention and your manuscript is so highly polished that she can see her reflection in your words. I'd query her myself but I just can't make myself write up a synopsis for the manuscript I re-revised last summer.
Then you've got Susanna Einstein who split with her partner and is going it alone. That's half the clients gone with the old partner, so why not got into line as a replacement?
The older, established agents can choose their clients because they don't really need to add to an already full stable. They'll go after the MFAs with a string of awards to their credit, and send you a quick rejection if they reply at all. Try hitting the agents who are hungry and possibly willing to take a chance on someone who is unpublished but shows talent.
Just make sure there are no typos or grammatical errors in anything you submit. No one is that desperate for a new client who requires an enormous investment of time to edit what has to be perfect before getting any kind of consideration.