The event marking the beginning of the current geologic era, the Cenozoic Era, is the Paleocene-Eocene Thermal Maximum (PETM). This occurred approximately 56 million years ago and was characterized by a rapid and significant warming event, a major extinction event, and shifts in ocean currents and marine ecosystems.

The PETM is thought to have been caused by a massive release of greenhouse gases, possibly from volcanic eruptions or the release of methane from the ocean floor. This rapid warming had a profound impact on life on Earth, leading to extinctions, changes in plant and animal communities, and the rise of new species, including the ancestors of modern mammals.

While the PETM is not the absolute beginning of the Cenozoic Era, it marks a significant change in Earth's climate and environment that ushered in the era we live in today.