Oldest living thing on Earth

An international team of scientists led by Sophie Arnaud-Haond may have found the oldest living thing on Earth:  a seagrass called Posidonia oceanica.  This Mediterranean marine plant is estimated to be tens of thousands of years old…depending on how you look at it.  Let me explain.

Seagrasses are clonal organisms.  That is, they don’t have to reproduce sexually like we do, but can simply give birth to identical copies of themselves.  This is common in microorganisms like bacteria, but also occurs in plants and even some animals.  It can be tricky to determine the lifespan of a cloned organism.  If the offspring is genetically identical to the parent, are they two distinct creatures?  This is doubly challenging when the offspring is simply a shoot that remains attached to the parent. 

Photographs of meadows of Posidonia oceanica by M. San Félix.

doi:10.1371/journal.pone.0030454.g002

In this case, the researchers examined a large clump of seagrasses measuring several kilometers across.  DNA testing indicated that much of the mass was made up of a single clone.  Because of the known rate of growth of P. oceanica, the scientists were able to determine that it must have taken that individual clone as much as a few hundreds of thousands of years to spread as far as it had.

There’s just one problem with this hypothesis. Sea level in that region was considerably lower 50,000 years ago. If this particular P. oceanica plant did indeed date back that far, it would have found itself living on dry land. Needless to say, this is an unlikely home for a marine plant.  More plausibly, the seagrass clone spread through a combination of growth and the dispersal of shoots and fragments to new locations.  If this were the case, the seagrass clone would still have to be several thousand years old.  That's still pretty darn old.

Of course, this begs the question of how you determine the age of a cloned organism.  We would have to find the very first shoot that started the colony to know that any part of it really had been alive for thousands of years. On the other hand, if we consider the entire colony to be one super-organism, then as a whole, it’s definitely extremely ancient.

What do you think?