The Most Recent Ancestor Of Vertebrates And Invertebrates

Nearly two centuries of experimentation and the enormous flurry of recent work, which has been only been touched on in the previous chapters, can be distilled into two very important but simple lessons about development and evolution. It is difficult to convey the great implica tions of these points in the history of the science of biology without some hint of melodrama. Thus, let us now consider these points and their profound implications.

The most visual of these two points is that we can now reconstruct a reasonably detailed image of the most recent common ancestor of vertebrates and invertebrates (Fig. 6.6). This wondrous creature who begat us all certainly had a well-defined head and tail with repeated segments in between, a belly and back, and basic tissue types such as nerve, muscle, and skin. It is likely that this creature also had some type of appendages or outgrowths from its body wall and a light-sensitive organ which served as the precursor to current-day eyes. It also seems clear from a variety of experiments similar to those described above that there are profound similarities between the formation of the rudimentary heart of invertebrates and the early stages of vertebrate heart development. In addition, the mechanisms by which neurons connect during the early stages of wiring the nervous system, trachea, or lungs branch, germ cells are produced, and basic immune system functions are based on common molecular machinery. Taking all of these similarities into account, our ancestor must have been a bilaterally symmetric animal that looked something like a shrimp. As this picture of our most recent common ancestor comes into ever-sharpening focus, it is apparent that this creature was the product of a great deal of previous evolution and must have lived at the same time as many other organisms, some of which may have eaten it routinely for a snack. A big question then is, Why was our ancestor the sole complex animal survivor of its time? Why did the progeny of the multitude of other animals that must have coexisted with our ancestor die out? We may never know the answer to this mystery, since these other organisms left no known trace either in the fossil record or as genetic material carried by their descendants.

Steven Jay Gould has popularized a similar massive extinction of life forms that occurred during the early Cambrian period. In his excellent book, Wonderful Life: The Burgess Shale and the Nature of History (1989), Gould tells the story of the Burgess shale fossil beds in which soft-bodied animals of the early Cambrian were preserved in exquisite detail owing to particularly favorable conditions of fossiliza-tion. This story of the early Cambrian period (which occurred long after the evolution and dominion of our sole surviving ancestor) is the first period in evolution in which large animals are adequately represented in the fossil record. The most interesting point of the Burgess Shale story is that among the many different basic categories of animals known as phyla which evolved in the late pre-Cambrian period, only a few survived the Cambrian extinction to give rise to modern-day animals. As an analogy to the explosion of life forms and its collapse during the Cambrian, Gould considers a bush that grows thick with branches during an ideal summer and then is severely pruned at the end of the growing season so that only a limited number of branches

Arthropods Vertebrates Common Ancestor
FIGURE 6.6. A possible representation of the most recent common ancestor of vertebrates and invertebrates.

are left to serve as shoots for growth in the following year. Following one of Gould's favorite themes, he argues that it may well be random as to which branches survive and which ones are trimmed. Whether or not the survivors of this cataclysm had something special to offer in the face of environmental adversity or were only the lucky ones who escaped the shrapnel of a comet impact, the main point is that the Cambrian extinction is one of several clear examples in which only a fraction of the organisms alive at a given time survived some type of devastating worldwide crisis. Luckily for us, one of the few survivors of that extinction was a primitive ancestor of vertebrates. If that nondescript creature had disappeared along with many of its comrades, perhaps some green juice-oozing creature with an exoskeleton would be writing this book now—or perhaps intelligent life, as we consider ourselves to be, would have never evolved at all.

Given that the Burgess Shale provides a clear example of a great diversification of life and a subsequent brutal pruning of that bush of life, it is reasonable to propose that a similar type of story took place during a previous event when our most recent common ancestor emerged as the sole triumphant survivor of its time. If we adopt Gould's view of the Cambrian extinction, we might wonder whether this animal was special or just lucky. We also should again thank our lucky stars that this animal survived, because complex animal life might have otherwise perished altogether, perhaps never to have been replaced. Another obvious question is, What kind of crisis wiped out all but one animal form? Was it a comet like the one that is suspected of bringing about the demise of the dinosaurs? Was it some dramatic climactic change like an increase in the level of oxygen? Or could it have been some biological innovation, like the invention of homeotic genes, that gave our ancestor and its descendants a great advantage over all other forms of life? According to this last scenario, there may have been no cataclysmic event at all, but rather life forms descended from our ancestor diversified with time and systematically displaced all other forms of life because of the advantage they all shared. We may never know the answers to these fascinating questions. Then again, who would have ever thought we could build as detailed a picture of our ancestor as we have in the last decade? I suspect that there are still several exciting installments of this story to come, so stay tuned.

The second major implication of the most recent common ancestor of vertebrates and invertebrates being a highly structured creature rather than some type of simple amoeboid form of life is perhaps less lofty than the first, but is more relevant to concrete matters such as our health. The fact that we and model experimental animals such as flies and worms have inherited the same molecular devices for accomplishing the equivalent basic biological processes means that we can get very valuable information from these model systems and apply what we learn to humans. In addition to the clear examples of shared genetic systems for patterning embryos, appendages, and eyes, there also are a growing number of examples of genes involved in human diseases such as cancer and Alzheimer's disease that also play important roles in fly development. Studies of flies and worms may also have important implications for longevity, since there are mutants in both worms and flies that have lifespans 30-60% longer than normal. The commercial implications of the deep similarities between humans, flies, and worms have not gone unnoticed. There are several new biotechnology companies organized around using flies or worms to make fundamental discoveries that might lead to marketable products for humans. This is also good news for researchers studying flies and worms, since we are all hopeful that the major federal funding institutions for science will continue to make basic research on model genetic organisms a high priority.

In many ways, the realization that we share deep similarities in fundamental developmental processes with creatures as lowly regarded as invertebrates is the last of a series of assaults on the sanctity and special station of humans. This systematic erosion of our image of ourselves as unique in the universe was unleashed by Copernicus and Galileo when they demonstrated that the sun, not the earth, was the center of our solar system, and has progressively grown less palatable. Darwin dealt another big blow to our self-image when he concluded in his masterpiece, On the Origin of Species (1859), that monkeys are our cousins and that all life forms including humans may have originated from a single common ancestor. The discovery that the genetic code and basic biochemical processes are common to all life forms, made in the middle of this century, confirmed Darwin's visionary hypothesis and demoted us firmly yet another notch. And now, it turns out that we are made in the same creepy mold as flies and worms. Can it get any worse? Our bruised human egos aside, I think that we are finally beginning to see where we fit into the larger scheme of nature, and I personally find that comforting rather than disappointing. We are, after all, the descendants of millions of generations of survivors. I do not see any tragedy in not being uniquely fashioned by the hands of an orchestrating god, but rather see our being sculpted from the likeness of a fly as just one chapter in the amazing story by which against all odds, we are still in the game after having traveled a very, very, long and winding road. This perspective is cause for great celebration and for doing everything possible to preserve the unbroken chain of life yet one more cycle so that future generations of wild-and-crazy creatures will be permitted to succeed us. Who knows, maybe one of our descendants will conceive of a frightening science fiction story where they get transformed into a primitive dull-witted human.

Vertebrate appendages, like those of invertebrates, are patterned along the three interrelated A/P, D/V, and P/D axes. The positions of vertebrate limb buds with respect to the primary body A/P axis are determined by segment-identity genes, which control expression of the secreted factor FGF. FGF initiates outgrowth of the limb bud and defines the orientation of the A/P axis of the limb primordium, and thereby the position of the primary A/P organizing center of the limb (the zone of polarizing activity or ZPA). The key patterning gene expressed in the ZPA is the Hedgehog morphogen, which is produced in a posterior domain of the limb primordium. Hedgehog protein diffuses from the ZPA and activates expression of target genes in a threshold-dependent fashion. One important target gene activated by moderate levels of Hedgehog is BMP4, the vertebrate counterpart of the Dpp morphogen in flies.

Despite the fact that appendages are thought to have evolved independently in vertebrates and invertebrates, patterning of the A/P axis in vertebrate limbs is quite similar to that of fly appendages such as the wing. In both systems, a source of Hedgehog is provided in the posterior region of the appendage and diffuses anteriorly to activate genes in a threshold-dependent fashion. An important target of Hedgehog in vertebrates and invertebrates is the morphogen BMP4/Dpp. Consistent with vertebrate and invertebrate Hedgehog proteins having equivalent functions, fly Hedgehog can mimic the effect of vertebrate Hedgehog as a ZPA signal. There also are notable similarities in how the D/V and P/D axes are established in vertebrates and invertebrates. The Notch signaling pathway plays an important role at the D/V border of vertebrate limbs in a structure known as the apical ectodermal ridge (AER) and in defining the corresponding boundary of fly appendages (e.g., at the interface between dorsal and ventral surfaces of the wing). In addition, the distal tip of vertebrate and invertebrate appendages is associated with the universal expression of a transcription factor called Distalless, which is essential for initiating P/D polarity in fly legs. These similarities in patterning the three organizing axes of vertebrate and invertebrate appendages suggest that a network of genes controlling early steps in patterning appendages or protrusions from the body wall existed in the most recent common ancestor of vertebrates and invertebrates and these genes have continued to exert similar functions throughout the evolution of vertebrate and invertebrate lineages.

Eyes appear superficially to develop by very different mechanisms in vertebrates and invertebrates. Surprisingly, however, formation of eyes in mice and humans depends on the function of a common transcription factor encoded by the pax6 gene, corresponding to the fly eyeless gene, which is required for initiation of eye development in flies. pax6 functions much like eyeless because misexpression of vertebrate pax6 in flies induces the formation of fly eyes in in appropriate locations. As in the case of appendage development, the underlying similarities in molecular mechanisms for initiating eye development in diverse species suggest that the most recent common ancestor of vertebrates and invertebrates possessed some type of light-sensing organ, the formation of which depended on the function of a pax6/eyeless gene.

The unanticipated underlying similarities between the establishment of primary body axes in vertebrate and invertebrate embryos (Chapter 5) and the similarities in mechanisms for patterning appendages and eyes in these two anciently diverged forms of life strongly suggest that the most recent common ancestor of current-day vertebrates and invertebrates had a well-developed body plan, appendages, and light-sensing organs. In addition, a variety of other work suggests that our most recent common ancestor had also invented early elements of a heart, trachea, reproductive cells, neural wiring, and basic immunity. These revelations permit us to reconstruct an image of this common ancestor, which most likely resembled a shrimp-like creature. Because this ancestor must have lived in a world filled with other organisms, a major question is, Why was this the only creature to give rise to surviving descendants? Was there some great cataclysm that eliminated all other life forms, or did our ancestor invent some revolutionary biological property that gave it a great advantage over all other life forms? Perhaps the future will hold the answer to this tantalizing mystery.

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Responses

  • dinodas
    What are vertebrates and invertebrates?
    2 years ago
  • tewelde
    What is most likely similar to the ancestor of these four classes of vertebrates?
    1 year ago

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