Over the past 10 years, fossils from earlier dinosaur species have been found in rocks between 100 and 176 million years old in Europe, North America and China.
The findings have confirmed that the Tyrannosaur family of dinosaurs comes from small-armed and long-bodied predecessors.
But, until now, there's been little evidence of a direct link between those dinosaurs and the huge T Rex.
Tiny T Rex relative is unearthed
Lead researcher Dr Paul Soreno, from the University of Chicago, said: "It's as close to the proverbial missing link on a lineage as we might ever got for T Rex.
"From the teeth to the enlarged jaw muscles, the enlarged head, the small forelimbs, the lanky, running, long hind-limbs with the compressed foot for hunting prey: we see this all, to our great surprise, in an animal that is basically the body weight of a human or 1/90th the size that ultimately this lineage would reach in T Rex at the end of the Cretaceous [period]."