Global warming fuelled an explosion of species diversity 500 million years ago that ultimately led to the human race, new research suggests.
During the "greenhouse interval", there were no permanent polar ice sheets and sea temperatures around Britain reached a balmy 25C.
Scientists made the discovery after looking for clues in tiny fossil shells embedded in 510-515 million-year-old blocks of Shropshire limestone.
This was a period known as the "Cambrian explosion", when representatives of all the major animal groups first appeared.
The huge surge in diversity set the scene for life to evolve into a multitude of complex forms including fish, reptiles, birds and mammals.
Without the Cambrian explosion, humans would not have inherited the Earth.
Scientists had suspected that the Cambrian was a warm period in the Earth's history, but proof was lacking until now.
The new evidence suggests it was a "greenhouse interval" - a time when high levels of carbon dioxide filled the atmosphere and temperatures soared. A similar climate existed during the reign of the dinosaurs.
Thomas Hearing, from the University of Leicester's School of Geography, Geology and Environment, said: "Because scientists cannot directly measure sea temperatures from half a billion years ago, they have to use proxy data - these are measurable quantities that respond in a predictable way to changing climate variables like temperature. In this study, we used oxygen isotope ratios, which is a commonly used palaeothermometer.
"We then used acid to extract fossils about 1mm long from blocks of limestone from Shropshire, UK, dated to between 515 - 510 million years old. Careful examination of these tiny fossils revealed that some of them have exceptionally well-preserved shell chemistry which has not changed since they grew on the Cambrian sea floor."
The oxygen isotopes - atomic strains of oxygen - indicated very warm sea temperatures ranging between 20C and 25C.
Climate simulations showed that such temperatures were likely to occur with the Earth in a "greenhouse" state.
Co-author Dr Tom Harvey, also from the University of Leicester, said: "Many marine animals incorporate chemical traces of seawater into their shells as they grow. That chemical signature is often lost over geological time, so it's remarkable that we can identify it in such ancient fossils."
The findings appear in the journal Science Advances.