Where Do Our Arms Come From?
Where do our limbs come from? This is a question that has puzzled scientists for more than 150 years. The emergence of shoulders has caused them particular headaches. Now, however, researchers have gained more insights thanks to the head of an ancient fish.
Wind back the clock 500 million years and you will come across our vertebrate ancestors: eel-like marine animals with no fins or limbs. All kinds of species would develop from them. First these had fins, then legs, wings and arms. How exactly those limbs came into being has been hotly debated for decades now. One of the greatest mysteries in this whole evolutionary journey surrounds how our fishy ancestors developed shoulders.
Fin fold & gill arch
Scientists have two main theories about the formation of pectoral fins - which were later to evolve into our own arms. The first is the "fin fold hypothesis”. It states that the pectoral fins developed from the body, specifically a bundle of muscle on the fish's flanks. Even if that is the case, it is not clear how shoulders could have come into being.
The other explanation is the "gill arch hypothesis”. According to this, pectoral fins developed from the head, specifically the gill arches (the bony structures that support fish gills). The shoulder then formed from this. This theory is difficult to prove, because the gill arches are composed of cartilage, which does not fossilise very well.
A recent study has now reconciled these two hypotheses, offering a little more clarity around the origin of our limbs.
Above, you can see the gill arches behind the gills.
A fossil with answers
To study these ancient beginnings, a research team analysed a fossil of a 407-million-year-old fish: Kolymaspis sibirica, one of the earliest species of jawed fish.
The fossil showed that one pair of gill arches had been gradually transformed over time into a joint at the back of the head. In the millions of years that followed, this structure then became "our" shoulder. How the fins then emerged is not yet entirely clear, but the researchers believe they were formed from parts of both head and body. In other words, both the gill arch and fin fold hypotheses are (partly) correct.
Now the researchers want to examine more types of fossil fish to gain a clearer understanding of the evolutionary story behind our limbs.