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Meanwhile, molecular studies of gene similarities between lancelets and today's vertebrates suggest that the vertebrate lineage goes all the way back to million years ago. After examining dozens of genes, the picture that emerges is that of a protovertebrate brain.
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In the lab, the Hollands apply an electric current to make the lancelets shed eggs and sperm, then the researchers raise the embryos. Conodonts, surprisingly, turn out to be full-fledged vertebrates, even closer to living jawed fish than to lampreys or hagfish. In the regions of the lancelet nerve cord where the Hollands found forebrain and midbrain genes at work, the neuronal structure matches that of the vertebrate forebrain and midbrain. Until very recently, the earliest undisputed vertebrates were a mere million years old.
These clues suggest that Haikouella is poised at the transition from invertebrate to vertebrate, closer to vertebrates than even the lancelet. The conclusion that bone was born after the rise of vertebrates is not yet certain, as more primitive chordates may turn out to have possessed the precursors of conodont mouth parts. Lacalli has found lancelet neurons whose structure and organization resemble those of vertebrate limbic neurons and that are located in the corresponding parts of the midbrain and forebrain.
And conodonts represent the first appearance of a mineralized skeleton. The vertebrate limbic system, which includes the hypothalamus, monitors the body's internal state, such as its temperature and hormone levels. Only after the rise of conodonts did the armored jawless fish, the ostracoderms, appear, and from one of their ranks, the jawed fish eventually evolved. But instead of a divided brain, one of the key inventions of early vertebrates might well have been a nose.
Last November, Chinese researchers reported a trove of specimens of a creature called Haikouella. These small, jawless fish with bodies completely covered in bony plates of armor are thought to have dined on sea-floor invertebrates and to have used their armor to defend against predators. Studies of living vertebrates reveal that as an embryo forms, a sheet of cells on its surface curls up into a tube that sinks into its body. In some ways these sliver-shaped impressions on ancient rocks look like lancelets, but they also have a few key vertebrate traits unnecessary for filter feeders, such as eyes and muscle blocks.
It would take over a decade, however, before they had the tools to do so. The fossil nerve cord has an even larger swelling than does that of the lancelet. Because it's the difference that becomes critical for understanding vertebrate evolution.
Some researchers have questioned this close kinship, noting that Haikouella has a few anatomical peculiarities, such as in the organization of its muscle blocks. Finally in the s new fossils began to emerge with the conodont elements lodged in soft tissue. The lancelet eye is too crude to form images, gottman validating communication but Lacalli suspects it can detect moving shadows of predators.
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Meanwhile, equally magnificent specimens of invertebrates such as Nautilus and giant clams are tucked away in a few smaller, less popular rooms scattered throughout the building. For other lancelet researchers, the wait is agonizing. They would also benefit from eyes to see prey and sophisticated control of their bodies to chase prey down.
But it does possess vertebrate traits such as gill slits, rows of muscle blocks along its flanks, and a notochord, a stiff rod of tissue that supports the nerve cord along its back. If these armored fishes represent the earliest vertebrates, they suggest that brains and bone evolved together. Later, the bone spread out to form a protective coat of armor, as seen in the early bony fish. Then in two researchers proposed a new theory that provided an intriguing answer to these puzzles.
We all have skeletons, complete with backbone and skull, and big, complex brains. These cells also migrate, but only as a sheet moving on the surface of an embryo, not as small clusters traveling inside it. He suggests that the common ancestor of vertebrates and lancelets used its protolimbic system to switch between its handful of behaviors, such as swimming and feeding. He claims that lancelets have a rudimentary limbic system. Such differences add further weight to Gans and Northcutt's idea that early vertebrates shifted from filter-feeding to predation.
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