Grow a Backbone
Did we get our backbones from animals like this?Lately I have been reading a book by Jeffrey Schwartz called Sudden Origins. In it Dr. Schwartz talks about the idea that species are not made gradually but instead just suddenly appear (in geological time anyway).
Reading the book was a bit like panning for gold. It was hard work and I needed to sift through a lot of silt but every now and then I got a nugget of gold. One such nugget was about where backbones may have come from.
Hundreds of millions of years ago, there was a time when no animals had backbones. These animals also weren't bilaterally symmetrical. This just means that their bodies weren't set up as two sided (think right and left halves of a human). Instead, they were more like sponges and starfish.
So how do you get from a starfish to a regular old fish? Seems like you'd need to make an awful lot of changes. You would– unless you were looking at the larval forms of some of these animals.
Some modern creatures that just sit on the bottom of the sea have a larval form that moves around. This is useful in setting up distant colonies.
What is interesting is that some of these larvae have bilateral symmetry– they have a right and a left side. And some of them even have the beginnings of a notochord on their back! So all we would need to do is somehow freeze the beast at the larval stage and you're halfway to something with a backbone.
Basically then, we need for the larva to sexually mature before it physically does. This sounds like a toughie but it isn't as hard as you might think. It is common enough that scientists even have one of their awful names for it– neoteny.
The classic example is the axolotl salamander. This animal usually stays a tadpole throughout its life– it never makes it to the salamander stage. But it still lays eggs. And when those eggs hatch, they give rise to more salamanders that will become sexually mature as tadpoles.
This sort of thing is pretty common with salamanders. But it may have happened in a lot of other cases as well. Other possible examples include dogs, who act like wolf puppies. And some relatively hairless and flat faced chimps (you and me).
And maybe even an ancient larva of some sea squirt-like thing lying on the ocean floor. How cool is it that we may all be descended from a juvenile sea squirt?
Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.
2 Comments
well looking at some of my neighbours can definetly see a resemblance with the sea squirt
:D
but what did the sea squirt descend from?
Lucky you..just your neighbors. I was seeing sea squirt in some of my more distant relatives!
Good question and a really hard one to answer! There is a great link on Vendian animals at http://www.ucmp.berkeley.edu/vendian/critters.html. This is helpful as it gives us some idea about how little we know about our 600-700 million year old sea squirt ancestor. Given the spotty fossil record of these beasts, the record of what came before is even spottier.
Perhaps genomics can help us identify the last universal common ancestor, the beast we all descended from. Quest had a cool piece on a scientists who is studying genomes to reverse evolution to find what the DNA of our ancestors looked like (see http://www.kqed.org/quest/television/view/547 for the TV story).
From http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookDiversity_7.html:
Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals (sponges in particular) show similarities to collared choanoflagellates as well as pseudopod-producing amoeboid cells.