Human Longevity Inc. raced to a billion-dollar plus valuation on the hopes that it could apply advances in genomics to improve medicine. A drastic “down round” financing this week suggests investors have questions about its own longevity.

The funding round values the company—co-founded by genomics pioneer J. Craig Venter in 2014—at around $310 million, according to a Human Longevity regulatory filing obtained by Lagniappe Labs LLC, whose Prime Unicorn Index tracks valuations of privately held companies. That marks an 80% decline...