Oo! A question that I can help answer in part!

For what it’s worth, I’m not a historian, but I do have a degree in history. I’m currently a law student in Australia, which has a similar Solicitor/Barrister split profession. I can’t speak to how the American profession developed, but I can talk about how the English/Welsh (and by extension, Australian) profession developed separately.

First off, currently in England and Wales, solicitors are actually entitled to appear before many Lower Courts, and it’s really only in the superior courts that barristers become truly necessary. However, the profession is still very split in certain jurisdictions in Australia.

The development of the split profession is largely something of a historical accident.^1 The distant ancestor to the modern barrister, the ‘Serjeants-at-law’, were an import of the Norman Conquest of the 11^th Century. As an aside, this is also why so many older English precedent is peppered with French. From at least 1216, English courts were beginning to limit the rights of audience to ’regular’ advocates’.

The Initial Split

Under King Edward I, the two separate branches were beginning to emerge. A pleading system was established, whereby specially trained serjeants would conduct legal arguments, while an Ordinance of the King placed legal representatives under judicial control, ending the clergy as lawyers in the Court.^2

Solicitors

Originally, Courts would require litigants to show up, plead their case, and receive judgement.^3 However, over time, courts relaxed these rules, and allowed litigants to appoint agents to appear and speak on their behalf. Obviously, lawyers did not appear out of nowhere as a profession, and these agents were not initially professionals.^4 The right to an ‘attorney’ was declared by Parliament in the 15^th Century.

As the number of cases and the amount of litigation increased in the 16^th Century, the number of terrible, unscrupulous and immoral solicitors increased at the same time. Therefore, in 1605, Parliament enacted the first of what we now would consider to be the solicitor’s standards and practices, requiring written statements for fees, and requiring written accounting for disbursements made on the client’s behalf.^5

Barristers

In England and Wales, barristers operate out of Inns of Court: The Honourable Society of Lincoln’s Inn, the Honourable Society of Gray’s Inn, the Honourable Society of the Middle Temple, and the Honourable Society of the Inner Temple. These trace their origins to the late 13^th Century, where legal professionals would live, learn, work and socialise together. These societies were the ancestors to the ‘Bar’.

Beneath the serjeants were the ‘apprentices-at-law’ and ‘utter barristers’,^6 who were recognised in 1532 as men ‘learned in the law’, and in 1590 required a ‘call to the bar of an Inn of Court’ as the minimum qualification for rights of audience before a higher common-law court. In 1596, the QC or KC (also known as a ‘silk’) rank of barrister was established, and by the 19th Century, no more serjeants were appointed. Barristers and ‘silks’ now comprised the entirety of the Bar.

The Formalised Split

However, from the 16^th Century onwards, the Privy Council, the Judiciary, and the Inns of Court themselves began excluding attorneys and solicitors from membership of the higher prestige inns.^7 Because of the way the judiciary works, only those who are ‘called to the bar’ are entitled to appear before the Court to argue cases. By excluding solicitors from membership of the Inns of Court, it essentially made it impossible for them to be called to the Bar, and so restricted the right of appearance to those barristers who were members of the Inns of Court.

Conclusion

It’s really this exclusion of solicitors and attorneys (who have since been combined) that solidified the split profession. Although we can see that the profession had been split to some extent from 1216 onwards, it wasn't made explicitly formalised until the Inns began excluding solicitors, preventing them from being called to the Bar, and removing their rights to appear before a higher Common-Law Court.

Sources

A note on sources: These are written from the perspective of American jurists and scholars. As such, there may be implicit bias in regards to the split profession. I was unable to find a British source when I ran an academic search, but I'm sure they are out there.

1. Judith L. Maute, "Alice's Adventures in Wonderland: Preliminary Reflections on the History of the Split English Legal Profession and the Fusion Debate (1000-1900 A.D.)" [2003] 71, 4 Fordham Law Review 1357, 1358 ('Alice's Adventures in Wonderland').

2. Ibid 1360, citing Robert Megarry, Inns Ancient and Modern 10 (Selden Soc'y 1972). See also Harry Cohen, "The Divided Legal Profession in England and Wales - Can Barristers and Solicitors Ever Be Fused?" (1987) 12 The Journal of the Legal Profession 7, 12 ('The Divided Legal Profession').

3. Ibid.

4. Alice's Adventures in Wonderland, above n 1, at 1360, citing Edmund B.V. Christian, A Short History of Solicitors 3, 9 (1896)

5. Ibid, 1361.

6. Ibid, 1366 citing Robert Megarry, Inns Ancient and Modern 10 (Selden Soc'y 1972).

7. Ibid. See also "The Divided Legal Profession" at 12, citing H. Kirk, Portrait of a Profession 18 (1976).