Astronomers have identified an ancient family of asteroids residing in the Main Belt (the area between Mars and Jupiter) that is nearly as old as the Solar System itself, yielding important insights into how the planets and asteroids formed. In the early Solar System, rocky material collected into mid-sized bodies known as planetesimals; some then coalesced to form the planets, whilst others settled into orbits in the asteroid belt. Four-and-a half billion years of collisions between those leftover objects have largely broken up the original planetesimals and produced generations of younger and smaller asteroid fragments. These fragments drift over time, making it difficult to identify families of related asteroids and determine the original planetesimals they came from. Here, Marco Delbo' and colleagues have identified a group of dark asteroids with related orbits as part of a previously unknown family, which they estimate to be roughly 4 billion years old, making it older than most known families. Combining this with other known families, the authors are able to determine the size distribution of the original planetesimal bodies. The results will help to differentiate between a traditional theory of asteroid formation, whereby planets grow incrementally from smaller bodies, and recent models that propose large planetesimals directly accrete pebble-sized rocks.