KEY TAKEAWAYSHumana shares are tumbling Monday morning after The Cigna Group said it is not pursuing a merger with its smaller health insurance rival.Negotiations by Cigna to buy Human fell apart last year but Bloomberg reported last month that the two companies had held informal talks about a deal, with discussions in the early stages.Cigna shares are jumping in premarket trading. Humana (HUM) shares are falling 5% in premarket trading Monday after The Cigna Group (CI) said it is not pursuing a combination ...