There is a life after death, of sorts, but only for a few days. A new study in mice and fish shows that after the animals have passed, hundreds of genes reawaken and become active. Far from being creepy science, this discovery could have important implications for people receiving organ transplants, offer clues to forensic experts and even rattle the current definition of death.

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Researchers from the University of Washington led by microbiologist Peter Nobel came across this remarkable find while working to understand how and when genes are active, reports Mitch Leslie for Science. Other researchers had noticed that some blood and liver tissue taken from human cadavers still showed the sign of active genes—that is, the genetic material encoding those genes was being read and transcribed into the molecules they contained the instructions for.

Intrigued by what happens to the body after death, Nobel and his colleagues systematically evaluated genes in mice and zebrafish. Previously, scientists assumed that gene activity would gradually wind down after death, but that is not exactly what the team found. They noticed that between 548 genes in the fish and 515 in the mice were still switched "on." Most genes ramped down their activity in the first day after death but some genes in the fish were still active four days after they died, the team reports in the preprint publication bioRxiv.

The genes still ticking away included some responsible for responding to stress and regulating the immune system. Others were those that are important for a developing embryo and aren't needed after birth. Noble tells Science that that particular finding about developmental genes was "jaw-dropping."

Leslie writes: "One possible explanation for their postmortem reawakening, the researchers say, is that cellular conditions in newly dead corpses resemble those in embryos."

Genes involved in cancer were also active. That result might explain why people who get a new liver, for example, have a higher rate of certain cancers than do people who don't receive transplants, reports Anna Williams for New Scientist. The organ dies a little and those genes might be switched on.

How does this happen? When tissue is injured, some cells die but they still retain enough chemical cellular energy to keep some functions going. This is very useful in jump starting recovery. The same process might be at work here.

The observations Nobel's team made could even provide a kind of genetic clock for forensics. By knowing what genes are and are not active a certain time after death, experts can better estimate how long a body has been dead. This kind of estimate is very important in criminal investigations but with current science, "we are not very good at it," forensic scientist David Carter of the Chaminade University of Honolulu tells New Scientist.

There are many points on the road to death that can mark when an irreversible course has been set. Historically, the measures that doctors use to know when someone is no longer alive have evolved. While this new finding isn't likely to budge the current protocols of determining brain death, it does make clear that the line between alive and dead isn't always as clear cut as we'd like to believe.