|Terra approcimately 60-70 million years ago|
Approximately 65 million years ago a sentient species formed an interstellar civilization in an area of space not far from Terra, or so it is hypothesized. They apparently had a habit of terraforming worlds and transplanting creatures from various "interesting" (to them that is) worlds and spreading them across a number of terraformed worlds, perhaps creating menageries or hunting preserves or simply tailoring the local biosphere to their tastes. They did not perform much genetic engineering (if at all), but rather engaged in planetary and biosphere engineering and life-form importation.
Now they are long gone. The vast bulk of their technology and cities were eventually reduced to nothing but anomalous concentrations of elements in the soil and the rock. None of their machines survived in an even remotely opertable form. Some of their artefacts - more commonly, trash - were fossilized; there are also fossilized bones which may or may not be fossiles of the "Gardeners".
However, their true legacy is not fosslized archeological artefacts but terraformed worlds and a wide spread of certain life-forms across multiple worlds. Some worlds failed and reverted to their pre-terraformation forms; others survived; all were changed by the passing of 60-70 million years. Some life-forms were extinct at the same time as the "Gardeners" or in the intervening millions of years; others survived virtually unchanged; yet others continued to evolve. And on at least one world, transplanted organisms reached sentience. The so-called Gardner Hypothesis suggests that the biological similarity between many of the known sentient and non-snetinet species across known space arises from their evolution from imported stock, some of which may have been Terran in origin - in its most radical form, this hypothesis claims that the Chuwak and possibly even Cicek and Reticulans have evolved from Terran animals imported to their prospective homeworlds approximately 65 million years ago.