Went hunting. According to The Sims Wiki (copy pasted): The Sims series utilizes a new, in-house game engine for each base game, with expansion packs and stuff packs being built on the same engine for that specific game. For example, The Sims 2 introduced a 3D engine with visual and technical advances over its predecessor, while its sequel, The Sims 3, brought in a newer version of the engine that incorporated further visual enhancements with a fully fledged texture streaming solution to accommodate the open world.
The full article is lengthy and makes for interesting reading.
For me, the visual clincher is that TS2 is fully 3D, while TS1 is not. His comment suggests two things. That both engines use the same computer language, so the coding would be similar if not identical for the same functions. That the new engine might have been developed from the earlier one, expanded and improved to handle the increased complexity and demands of the new game. Common foundation but still a different engine.
