Excerpt from Some Assembly Required by Neil Shubin, plus links to reviews, author biography & more

Recently, researchers at Cornell University revisited the com­parison of swim bladders to lungs, using new genetic techniques. Their question: What genes help build fish swim bladders dur­ing development? In looking at the catalog of genes that are active in fish embryos, they found something that would have pleased both Dean and Darwin. The genes that are used to build swim bladders in fish are the same ones used to make lungs in both fish and people. Having an air sac is common to virtually all fish; some use them as lungs, while others use them as buoyancy devices.

Here is where Darwin's answer to Mivart becomes so pre­scient. DNA clearly shows that lungfish, Saint-Hilaire's bichirs, and other fish with lungs are the closest living fish relatives to land-living creatures. Lungs aren't some invention that abruptly came about as creatures evolved to walk. Fish were breathing air with lungs well before animals ever stepped onto terra firma. The invasion of land by descendants of fish did not originate a new organ—it changed the function of an organ that already existed. Moreover, virtually all fish have some kind of air sac, whether lung or swim bladder. Air sacs shifted from being used for a life in water to later enabling creatures to live and breathe on land. The change did not involve the origin of a new organ; instead the transformation was, as Darwin said more generally, "accompanied by a change of function."

Excerpted from Some Assembly Required by Neil Shubin. Copyright © 2020 by Neil Shubin. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.