As I mentioned a little while ago I have been made an associate lecturer for first year biology at the University I have been doing my PhD at. Most of my job is taking tutorial classes and the other week during one of these classes I got totally distracted by a very interesting discussion.
Part of the tutorial concerned the requirements and characteristics of life. The students had covered the requirements as being HOMR; Homeostasis, Organisation, Metabolism and Replication. These requirements are pretty self explanatory but we also suggest they know evolution (primarily as it still fits into HOMER) and the other major acronym, MRS GREN (Movement, Respiration, Sensitivity, Growth, Replication, Excretion and Nutrition), illustrated here as a video, and think of them as hallmarks or common attributes of life. For the purposes of this course, if it satisfies HOMR then its 'alive'.
In any case after defining HOMR and giving a few grey area examples like viruses (nope, not alive), dormant bacteria (depends) and parasites in a non-replicative host (sometimes) we were about to move on when one of the students piped up, “what about zombies?”. The whole class stopped. It was interesting to see them all pause and try to work out if she was joking. After a few seconds and the first few giggles had subsided I asked the student to clarify what she meant. She replied with “you know, zombies, like in the movies. Would you class zombies as technically 'alive'?”
My first question was what is a zombie? Most people know zombies as the undead after bbbrrrrrraaaaaiiiiinnnnnnnsssssss but many don’t realise that there are many types of zombies.
First we ruled out the Evil Dead canon of zombies which are raised from the dead by voodoo magic. While this was a silly extension of a concept there was no need to leave science behind all together.
What we are left with are the other canon of zombies like those depicted in 28 Days Later or Zombieland. These are zombies that develop as a result of some sort of pathogen infection. There is a great article that came out a little while ago that considers what a zombie pathogen would have to be capable of, but rather than be concerned with the pathogen, we were more concerned with the host.
Pathogen based zombies could be classed as the living-dead, the host never died and its actions are driven by the pathogen, somehow. Alternatively the pathogen kills the host and somehow drives its reanimation, the so called un-dead. While different, it is these kinds of zombie that we considered in my class.
So after defining the zombie we started to consider the requirements of life. Do zombies maintain homeostasis? We came to the conclusion that the living dead do but the un-dead cannot. The decay would be due to an inability to respond to changes and maintain an internal environment and a creature that has died would, according to the canon, be unable to heal or repair. The living dead however are living but mindless hosts hijacked by the pathogen and there is no reason they couldn’t continue to maintain an internal environment. In fact it could even be important for such a pathogens survival to maintain the internal environment of its host.
|We might need this later|
O is for organisation. A pathogen would have to be organised and complex in order to pull off the living death of its host and for the host to persist it must retain its cells to tissues to organs to organisms organisation. So as a class we took this one as a given.
Metabolism was easy. Um hello? They crave brains and eat them to provide sustenance. That's good enough for me.
It again seemed pretty reasonable to us that the cravings of a zombie could be irrational, especially if it was a mindless host and cells and pathogens need energy otherwise the living dead become the dead dead.
Finally, replication. I had always been taught that replication specifically meant of the genome and it is here that our host ceases to satisfy HOMR. Replication of zombies occurs via a bite but it does not result in replication of the host’s genome. The replication is only of the pathogen causing the zombification.
|...and yet there are zombie children. Is there zombie sex I wonder. I wonder but I'm not going to Google it. Photo credit: D Sharon Pruitt|
So zombies failed HOMR but what started as a weird question from an interested student turned into a memorable discussion that cemented these ideas into my students. But to make up for the disappointment of the failure their idea I decided to go looking for zombies…
I’ll leave that bit till next time.
Jump straight to Part 2.
Jump straight to Part 2.