Alphabet shares tanked 5% on Wednesday after four of Google's most decorated AI researchers announced their departures on the same day. Jeff Dean, who spent 27 years as the company's chief scientist, is leaving. Demis Hassabis steps back from running Google DeepMind to become chairman instead. The exodus sent a sharp message to investors about cracks forming at one of tech's most critical outposts.

The stock dropped hard and fast. It traded at $381.81 before the news broke, then nosedived to $355.16 within an hour. That 7% swing wiped nearly $260 billion from Alphabet's market cap at the low point. By afternoon the shares settled around $360.71, closing down 3.9% from Tuesday's finish of $375.35. The slide erased close to $190 billion in value from a company worth $4.61 trillion. Alphabet remains about 11% underwater from its 52-week peak of $404.47.

This isn't the first time. In June, the stock dropped 5.1% in its worst session of the year when two researchers quit days apart. Noam Shazeer jumped to OpenAI. He was one of the eight authors on the 2017 transformer paper, the architecture that powers almost every chatbot running today. John Jumper, who shared the 2024 Nobel Prize in Chemistry with Hassabis for work on protein structure prediction, moved to Anthropic instead. Now the other laureate has stepped back from the lab.

The four departing researchers are launching Discovery Loop, a Public Benefit Corporation focused on automating machine learning and scientific research. Dean co-founded the venture with Sanjay Ghemawat, Oriol Vinyals and Quoc Le. The group includes three of the most-cited names in AI research. Two rank among the most-cited in distributed systems as well. Dean arrived at Google in mid-1999 as employee number 30. He and Ghemawat built MapReduce in 2004 and Bigtable in 2006, the systems that let Google index the web at massive scale. Walking away after that legacy carries weight.

Koray Kavukcuoglu takes over running Gemini, Google's flagship AI product line. The leadership shake-up leaves open questions about continuity and vision at a moment when AI development moves at breakneck speed. Investors are watching whether the departures signal deeper problems or simply a natural churn of talent at the highest levels.

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