Prior To The Cambrian Explosion, All Large Animals Had What Characteristic In Common?

Abstruse

The Cambrian explosion is an excellent example of a 1000 idea that has been tempered by the steady collection of data to test hypotheses. Historically, the idea of an "explosion" developed from an apparent lack of bilaterian beast fossils before a certain point in the fossil tape, in dissimilarity with a great diversity of life that seemed to appear in the Cambrian catamenia. Deoxyribonucleic acid molecular clock estimates contradict this story, nevertheless, with well-nigh dates for the difference of major phyla predating the Cambrian by 100 1000000 to 400 million years. The contradiction might exist rectified past corrections to the clock or by discoveries of Precambrian bilaterian fossils. Although many candidates exist, no single ecology or biological caption for the Cambrian explosion satisfactorily explains the credible sudden appearance of much of the diversity of bilaterian animal life. Scientists' understanding of this phenomenon has been greatly amplified in recent years by better geological dating and environmental characterization, new fossil discoveries, and by a great expansion of our knowledge of developmental mechanisms and their evolutionary meaning.

The term "Cambrian explosion" refers to a hypothesized time when bilaterally symmetrical (bilaterian) animal groups of various forms diverged from a common ancestor during the early part of the Cambrian period, a geological catamenia starting almost 542 1000000 years ago (Ma). true, this would surely be ane of the most momentous times animal history, when the stage was set for the evolution of most of the ensuing diversity of fauna life, including the extant phyla. We would owe to this time the origin of mollusks, arthropods, and even our own major group, the Chordata. We tend to see the major groups of bilaterians as members of distinct torso plans, each belonging to its ain phylum. The phyla tin exist organized into two major groups, the Deuterostomia (echinoderms, chordates, and others) and the Protostomia (annelids, mollusks, and others), whose differences can be diagnosed by both molecular and morphological characters. The protostomes can exist subdivided into two major groups, the Ecdysozoa (arthropods, nematodes, and others) and the Lophotrochozoa (annelids, mollusks, nemerteans, brachiopods, and others).

The Cambrian-explosion hypothesis claims that this fantastic creature menagerie diverged from a common antecedent and go a recognizable set of body plans in a mere 20 million years or so. The earliest Cambrian—marked by burrows and small, strange, shelly fossils—culminates in a spectacular array of forms by about 520 Ma. A somewhat softer version of the hypothesis allows for divergence a few 1000000 years earlier the Cambrian, with an explosion of big-bodied organisms in the Early Cambrian.

The history of this idea is as fascinating as the idea itself (see Levinton 2001). By the 1830s and 1840s, a succession of rocks in Wales and England revealed a series of animal forms, with the newest rocks containing forms that strongly resembled living animate being species, and the oldest including a series of strata that patently lacked recognizable beast fossils. Soon thereafter, a great controversy arose between Adam Sedgwick, of Cambridge Academy, and Roderick Murchison, of the Geological Lodge of London. Sedgwick proposed the existence of a sequence of rocks in Wales, which he named Cambrian (and where, at showtime, no fossils were constitute). The controversy with Murchison was over the verbal sequence of rocks from the Paleozoic era. By the 1870s, the idea of a Cambrian, the oldest geological period with animal fossils, was widely accustomed.

Charles Darwin recognized the implications of the Cambrian for his ideas on evolution. Although the beginning of animal life appeared to occur in the Cambrian period, Darwin (1859) idea that the fossil record might have failed to record a long preceding time of gradual unfolding of animal life. After all, within the Cambrian, there were well-formed and recognizable brachiopods, trilobites, and other groups that could readily be assigned to major groups of animals. Could these well-formed fossils accept sprung from the inchoate, amorphous likely ancestors of brute life with no intermediates? Hardly probable, argued Darwin. He explained:

I look at the natural geological record, as a history of the earth imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating but to two or three countries. Of this book, only hither and in that location a short chapter has been preserved; and of each page, only hither and there a few lines.... On this view, the difficulties above discussed are greatly diminished, or even disappear. (Darwin 1859, pp. 310–311)

If Darwin set the stage for the Cambrian-explosion hypothesis, the cast of zoological characters was elaborated through the momentous discoveries of Charles Walcott, whose great achievements as a Cambrian specialist were matched by his accomplishments in government, leading eventually to his leadership of the U.s. Geological Survey, the Smithsonian Institution, and the Carnegie Institution of Washington (Yochelson 1996). Every bit a geologist, Walcott established the modern framework for the trilobite-based bio-stratigraphy of the Cambrian period in North America, which arose from many seasons in the montane west and from painstaking work in the laboratory. Just he is remembered mainly for the discovery of the Burgess Shale and his clarification of a menagerie of animate being life that could not be imagined to occur in the otherwise much less various surrounding Cambrian formations, where few species other than trilobites and brachiopods were preserved. Walcott and his party, which included his wife and children, discovered scores of brute species, many of which were delicate soft-bodied forms, preserved every bit organic films on the shale surfaces. Considering the remoteness of the site, virtually Yale, British Columbia, Walcott'south personal effort and leadership produced the greatest fossil-collecting achievement in the history of invertebrate paleontology (Gould 1989, Yochelson 1996).

What was and so spectacular about Walcott's discovery? In one case the fossils he identified (priapulids, annelid worms, crustaceans, and the similar) were cataloged, alongside an as various group of fossils afterwards described by paleontologist Harry Whittington (1985) and his colleagues, scientists could say with some confidence that many of the living major groups of animals had appeared by the Eye Cambrian (the age of the Burgess Shale, ca. 505 Ma). Whittington'due south discoveries expanded the catalog of forms resembling living groups, but information technology also added a startling array of weird creatures, some of which could not be related easily to any of the known phyla. What could be more spectacular than the formidable predator Anomalocaris (figure i)? And what could exist more impenetrable than the classification of genera such as Opabinia and Wiwaxia? Later discoveries of rocks with like levels of fossil preservation to that of the Burgess Shale—from sites in Greenland; in Chengjiang, southwestern China; and in other localities—extended the fourth dimension of origin of these groups to the Early Cambrian and added nevertheless more multifariousness to this early apparent explosion of fauna life, witnessed past the fossil record.

Effigy 1.

Some animals of the Burgess Shale that are rarely preserved in nearby contemporaneous Middle Cambrian rocks. (a) Anomalocaris, systematic status unclear, upwards to. 0.5 meters long. (b) Aysheaia, onychophoran, 1 to half-dozen centimeters (cm). (c) Sidneyia, arthropod, 5 to xiii cm. (d) Ottoia, priapulid, ii to sixteen cm. (eastward) Naraoia, ii-lobed noncalcified trilobite, 9 to xl millimeters (mm). (f) Pikaia, chordate, 4 cm. (k) Olenoides, trilobite, 50 to 85 mm. (h) Opabinia, status unknown, 4 to 7 cm. Reprinted from Levinton (2001) with the permission of the Smithsonian Institution.

If Darwin established the theater and Walcott and Whittington gave us the cast of characters, it was Preston Cloud who wrote the first typhoon of the play that has guided all thinking nearly the Cambrian explosion in recent decades. Like any great playwright, Cloud offered a clear-headed rethinking of a complex state of affairs and focused his audition'southward thinking on a few great ideas. He had predecessors, but Cloud managed to capture the idea of the Cambrian explosion with the greatest eloquence and geological sophistication.

Cloud was trained every bit a stratigraphic paleontologist at Yale and later rose as a scientist in the Us Geological Survey. Aside from his bang-up leadership and mentoring of a generation of paleontologists, he adult an integrative approach to paleontology, calculation skills in paleogeography, carbonate stratigraphy, and carbonate sedimentology. His later career at the University of California, Santa Barbara, widened his interests to astrobiology and the origin of life. His observations equally a paleontologist led him to characterize the Phanerozoic fossil record as a series of evolutionary eruptions, with the Cambrian being the greatest of all (Deject 1948). But it was Cloud's 1966 Yale lecture that solidified the modern approach to the Cambrian explosion. In the remarkable, long paper that resulted (Cloud 1968), he unified and so-innovative studies relating reconstructions of ancient global climate to the Cambrian appearance of animal life. First, and foremost, he insisted that we invest our efforts in the evidence for the identity and age of aboriginal fossils: "Is it surely a fossil or the piece of work of an organism?... And is it surely owned to rocks whose stratigraphic position is such that they cannot reasonably exist included in the Paleozoic?" (Cloud 1968, p. 51). Given the prove, he concluded, "the appearance of multicellular brute life in the Cambrian may actually have been nearly every bit sudden as the record suggests, an case of eruptive development of the root stock of animal evolution itself" (Cloud 1948, p. 346).

Cloud emphasized the demand to find a link between a change in the global environs and the Cambrian evolutionary eruption. His original emphasis on dissolved oxygen, based on show from banded iron formations, has not withstood the test of fourth dimension; but his emphasis on evidence has been a cornerstone of Cambrian studies.

The more contempo major paleontological discoveries accept intimately related studies of phylogenetic relationships of early bilaterian groups to great refinements of the geological timescale. One must remember that in the 1960s, the error associated with stratigraphic correlation of geological sections and the error produced from radiometric dates produced uncertainties of millions to tens of millions of years in Cambrian and Ordovician time. The most recent geological timescale (Gradstein et al. 2005) shows vast improvement, and the current judge of the first of the Cambrian at 542 Ma is virtually likely accurate to one million years. The most startling result is the credible rapid advent of almost of the animal phyla, which can be bracketed within a fourth dimension frame of approximately 20 million years or less (Bowring et al. 1993).

Evidence on the origin and deviation of bilaterians

The evidence on the origin and divergence of bilaterians falls into 4 categories: (i) molecular clock data, (2) fossil data on the appearance of bilaterian groups, (3) morphological and phylogenetic written report of the fossil record; and (iv) genetic evidence.

Molecular clocks.

Molecular clocks balance on the presumption of a human relationship between the time since two lineages have diverged and the degree of genetic difference between them, based on the idea of integrating evolutionary rate with time. Time can be estimated past taking ii sister evolutionary lineages, A and B, and finding dated fossils of each grouping. Genetic distance is a measure out of deviation betwixt the number of nucleotides in two Deoxyribonucleic acid sequences or the number of amino acids in two protein sequences. If the fossil record were consummate and genetic divergence were at the aforementioned constant rate for all genes over the time since the split, this process would be easy. Rates of divergence could be established, and and so one might extrapolate the rate to explicate the time of divergence for two lineages that are aboriginal and whose fossil origin dates are unknown. Only the fossil record is commonly incomplete, and corrections for genetic departure must be made for at least the following possible biases: (a) differences in rates of divergence among genes, which make some genes evolve too rapidly to preserve phylogenetic information and others evolve too slowly to give enough sequence modify to properly resolve what might have been very rapid splitting events concentrated in a curt period; (b) differences in rates of departure in different lineages on an evolutionary tree; (c) possible differences in rates of genetic divergence over fourth dimension (e.g., different rates of evolution under certain ecological, ecology, or evolutionary situations); and (d) heterogeneity of rates of change at different parts of a Deoxyribonucleic acid molecule. These biases have led to a number of studies that attempt to correct for rate heterogeneity in different parts of a tree or simply to drop those cases where such heterogeneity exists. Some studies take attempted to contain large numbers of genes, which might boilerplate out the variation by the law of cardinal tendency.

All major studies consistently produce a date of divergence for the protostomes and deuterostomes considerably before the outset of the Cambrian (Smith 1999, Levinton 2001). More than recent studies accept used more genes but have yielded a wide variation of dates (figure ii). Consistently, however, these dates are Precambrian. If all of the major phyla diverged in a very curt menses of time, we might expect the issues in reconstruction that have been encountered, since closely spaced nodes hundreds of millions of years former would exist nearly impossible to resolve (Levinton et al. 2004).

Figure two.

Left: Major fossil occurrences near the beginning of the Cambrian period. Right: Estimates of the divergence fourth dimension of the protostomes and deuterostomes (i.e., bilaterian animal phyla) derived from various analyses of molecular sequences (Smith 1999, Levinton 2001).

There is still great disagreement over methods and approaches. Earlier studies using few genes (e.1000., Wray et al. 1996) have been criticized for including few genes with too much heterogeneity of rates over trees, merely the substitute studies by critics accept failed to produce dates consequent with the Cambrian and have also failed to produce dates that are highly consistent amidst genes (e.g., Ayala et al. 1998). Peterson and colleagues (2004) concluded that previous studies had used genes with rates of evolution inappropriate for studying most of the phyla participating in the Cambrian explosion, but their correction however produced dates that preceded the Cambrian by xxx one thousand thousand to 114 one thousand thousand years. Blair and Hedges (2005) reconsidered this most juvenile of Cambrian bilaterian divergence estimates and plant that they more often than not derived from a selection of fossil calibrations that biases results toward slower rates of divergence, and not toward different rates of molecular divergence between vertebrates and invertebrates, every bit claimed by Peterson and colleagues. Employ of a different fossil calibration led to a corrected range of divergence times, 777 Ma to 851 Ma. With a rapid increment in sequence evolution at the kickoff of the radiation, a regular, constant molecular clock might overestimate the divergence time. Simply picking the highest charge per unit of sequence evolution would still push the departure time to no more than recently than 586 Ma (Bromham and Hendy 2000). Thus, although the large range of divergence time estimates does not inspire confidence, we must still face the current decision that molecular estimates do not square with the fossil occurrence information, which places the groovy radiation between approximately 540 Ma and 520 Ma. At present, it is likely that the assumptions of the models of molecular evolution may influence the outcomes too strongly to allow any meaning confidence in estimates of molecular dates for the divergence of the Bilateria (Welch et al. 2005).

The Cambrian fossil flare-up.

The fossil data support a decision at variance with the molecular clock estimates. The bilaterian animal groups seem to appear in the fossil record at or just before the first of the Cambrian. In the past 15 years, members of more and more than phyla and major bilaterian classes, including vertebrates (Shu et al. 1999), take been plant in rocks dating dorsum to the Early Cambrian (Levinton 2001). The literal interpretation of the fossil record would suggest a complete divergence of the bilaterians in about 20 million years or less.

Trace fossils, which are burrows and trails recorded in the sediments, appear in a burst most the base of the Cambrian. Much of this diversity is dominated by burrows that served as shelters from which infaunal animals fed or moved toward the sediment surface (Dzik 2005), but arthropod traces also go prominent in the earliest Cambrian (MacNaughton and Narbonne 1999). The rising of bioturbation at the cease of the Tardily Proterozoic (McIlroy and Logan 1999) may have been responsible for the devastation of microbial mats, which had dominated the sediment surface in the Ediacaran along with some horizontal traces. Deep burrowing, probably a response to surface predators, is non well recorded until the Ordovician and even subsequently.

Information technology is always an open question whether or not the apparent sudden appearance of bilaterians in the Early on Cambrian results from a preservation gap. It might well exist that rocks inappropriate to preservation boss the fourth dimension earlier the Cambrian. Ediacaran fossils in the latter part of the Proterozoic are abundant, but they are commonly found in sand, which would not preserve the delicate structures seen in the organic films discovered in Lower Cambrian finer-grained sediments. With the lack of dissolved oxygen—or perhaps of mineralized skeletons—before the Cambrian, bilaterians might take been quite pocket-size in body size, which would reduce the probability of preservation (Levinton 2001). Certainly the special Burgess-Shale type of frail preservation is lacking in Precambrian rocks younger than 750 Ma to 850 Ma (Butterfield 1995), which leaves a considerable gap in time until the Early Cambrian occurrences.

Morphology and phylogenetics revealed by fossils.

Possibly the strongest bear witness to support the Cambrian evolutionary explosion of animal forms is the kickoff clear advent, in the Early Cambrian, of skeletal fossils representing members of many marine bilaterian animal phyla. (Simply the Bryozoa and then far elude discovery in the Cambrian, but they are found in the Ordovician.) The impression of an explosion is heightened past a number of fossils with unclear affinities to extant phyla. At beginning, it was claimed that the Early on Cambrian is replete with forms that accept no obvious resemblance to extant phyla or even to other ancient groups (Gould 1989). Some species have characters that may place them as ancestral members of extant phyla (Conway Morris and Caron 2007), but controversy exists as to groups such as the halkyerids (Vinther and Nielsen 2005).

A well-known taxonomic bias crept into studies of Cambrian and other early beast fossils. When a foreign fossil was found, unclassifiable body parts influenced paleontologists to classify such organisms as members of new classes of extant phyla or even new phyla. Thus, a serial of descriptions resulted in 21 named classes of the phylum Echinodermata (Levinton 2001). Ironically, this is precisely the reverse of what Gould (1989) argued was the declining of the great paleontologist Walcott, who supposedly tended to ally the strangest of organisms to conventional groups that had already been described. Gould may have been correct about Walcott, but he missed the residue of the picture. With gay abandon, paleontologists were naming early on animal taxa and defining them equally members of new phyla or classes. In effect, paleontologists are rewarded with recognition for discovering a new taxon when they assign it to a higher level of classification. (Wouldn't you rather discover a new phylum than a new species of an existing genus?) The trend was accelerated with the second great investigation of the Burgess Shale by Harry Whittington and his colleagues. A weird, spiky, worm-like fossil was whimsically named Hallucigenia and thought to be a taxon unrelated to conventional known phyla (Conway Morris 1977). Another fossil, previously idea by Walcott to be an annelid, was re-described as belonging to a new phylum, perhaps related to mollusks (Conway Morris 1985). This bias forced a notion of an evolutionary lawn, in which numerous unrelated taxa appeared suddenly in the Cambrian (and the Ordovician, in the case of Echinodermata), which fit nicely with Deject's (1968) concept of the polyphyletic origin of the animal phyla.

2 of import breakthroughs changed scientists' conception of a Cambrian explosion as an evolutionary lawn of strange and unrelated shoots: (one) reexamination of the morphology of these "strange" creatures and (2) afterthought of these disparate taxa as members of an evolutionary tree, which represents the morphological characters of different groups from the betoken of view of evolutionary relatedness. Many of the supposed oddball echinoderms, for example, were mistakenly classified as avant-garde, differentiated forms. Instead, they could be assigned to bequeathed locations on an echinoderm evolutionary tree. Thus, the evolutionary lawn of echinoderms was transformed into a far more sensible evolutionary tree (Smith 1984). Second, a reexamination of characters began to show that other "oddballs" were non then strange, after all. The supposedly weird Hallucigenia was shown to be reconstructed upside downwardly. It was unlikely that this worm sabbatum on spikes, which instead projected upward to protect against predators. More deflating was the discovery that Hallucigenia was a mundane member of a larger Cambrian fossil group, the Lobopodia, related to living velvet worms (Ramskøld and Xianguang 1991). The upshot was something like being in a dream and seeing a political party of weird, colorfully dressed Harry Potter characters, simply to wake up and realize that you were looking at your ordinary friends, wearing blueish jeans and T-shirts.

The result of this new approach has been very important for comprehending the relationships of early on beast life, simply we are but at the offset of an understanding. Researchers are carefully examining the characters of fossils and constructing evolutionary trees, which leads to salubrious exchanges of views.

The utilise of advisedly reckoned organismal characters and the construction of phylogenetic relationships have produced some tantalizing results. A major nomenclature of fossil and living representatives of the phylum Arthropoda, based on morphology, has presented a difficult conundrum, maybe the Achilles heel of the Cambrian-explosion hypothesis. A complete analysis of the evolutionary relationships of the arthropods demonstrates that trilobites are not an bequeathed group, but rather are derived (that is, distant from ancestral nodes) in location on the evolutionary tree (Briggs and Fortey 1989). This finding becomes quite intriguing when ane realizes that the first advent of the trilobites not but defines the Early Cambrian advent of arthropods at the base of the Atdabanian only occurs with the trilobites already deployed into 2 large-scale biogeographic realms (Fortey et al. 1996). In other words, at the very beginning of known fossil arthropod (and other bona fide bilaterian) preservation, advanced arthropods are already present and bigeographically differentiated.

Moreover, an emerging moving-picture show of Early Cambrian arthropods suggests that primitive forms, bearing characters ancestral to many euarthropods (Waloszek and Maas 2005), coexisted with definitive crown-grouping (pregnant that they have characters of derived groups) crustacea, a modernistic group that dominates the oceans today (Zhang et al. 2007). Arthropods described from southern China, and three-dimensionally preserved Orsten-blazon fossils from the Lower to the Upper Cambrian, demonstrate a remarkable coexistence of dissimilar stages of evolution, from ancestors to derived groups such as crustaceans and their sister grouping (Siveter et al. 2001, Hou et al. 2004, Waloszek et al. 2007). This remarkable coexistence leads to the inevitable conclusion that, even by the Early Cambrian, arthropods were very diverse and comprised a large number of lineages in various evolutionary positions of ancestral and advanced condition. This includes a crown-group crustacean of large body size with a sophisticated particle feeding mechanism (Harvey and Butterfield 2007). Could this have happened in a geological "instant"?

There is no surprise in all of a sudden seeing at the base of the Cambrian a mixture of ancestral and derived forms. Fifty-fifty in our living biota today, we have a surprising range of bequeathed forms (sometimes called "living fossils") circumstantial with highly derived forms in almost every phylum. But of form the co-occurrence of a panoply of ancestral and derived forms is non testify that everything happened at once. Ane cannot escape the conclusion that something is not preserved, or is yet to be found from the fossil record, from before the first occurrence in the Early Cambrian of trilobites and truthful crustaceans, allow lone other bilaterians. I admit that, as Carl Sagan once said, the absence of evidence is not strong show of absence. Only it does suggest that Cambrian explosionists take some work to practice, as do their opponents.

Are there bona fide Precambrian bilaterian fossils? This has been a road littered with difficulty and disappointment. Although Cloud (1968) systematically discredited near all described Precambrian bilaterian fossils, he was unable to discredit an annelid-similar fossil plant in 700-million-to 900-million-year-old rocks in People's republic of china (Cloud 1986). Some tantalizing fossils that might be bilaterian have been plant in the latter part of the Proterozoic, known as the Ediacaran (Fedonkin and Waggoner 1997), and bilaterian-like embryos have been plant in the Ediacaran Doushanto Formation in Prc (Xiao et al. 1998). None of these tin can easily be placed on a tree of known bilaterian groups. A possible sis grouping to the trilobites has been described (Fortey et al. 1996). A large menagerie of fossils was found first in southward Australia (Glaessner and Wade 1966) and later worldwide in Ediacaran-aged rocks. These fossils appear to belong to the Cnidaria and other groups of uncertain status. A recently discovered trace-similar fossil, claimed to be i billion years sometime, may belong to a bilateral organism, but not necessarily a bilaterian animate being (Bengtson et al. 2007).

Genetic questions.

The early origin and conservatism of major functioning parts of the bilaterian genome accept been then well documented that they are at present textbook truisms. Well-nigh intriguing is the continuance of the Hox gene complex, whose gene lodge and activity combine to determine inductive-posterior specialization (McGinnis et al. 1984), which is the very essence of existence a bilaterian. Information technology has been suggested that the emergence of Hox genes allowed the diversification of the Cambrian explosion (Erwin et al. 1997), merely this is not likely if all bilaterians shared this specification machinery. The diversification would accept to be acquired by genes downstream of Hox genes or by other genes entirely such as various transcription factor genes and cell-signalling genes. The two major bilaterian animal groups, the Protostomes and the Deuterostomes, also share genes that are crucial in the evolution of eyes, circulatory systems, skeletons, and many other systems (tabular array 1; Levinton 2001). These genes advise the possibility of an ancestral creature deep within the Precambrian that is mobile, with a established genetic mechanisms determining anteroposterior development and the adequacy of forming eyes, a nervous system, a circulatory system, and a skeleton (the latter owing to the presence of lysiloxidase). In other words, the genetic capability of developing a complex mobile bilaterian beast exists deep within Precambrian time, and the show even indicates a radiation of disparate groups that have non all the same been found.

Table 1.

Important developmental genes, proteins, or genetically specified systems found in both Protostomes and Deuterostomes, which together establish the bilaterian beast phyla, or in a presumed mutual ancestor.

The plot thickens. The recent completion of the draft genome of the sea anemone Nematostella vectensis (Putnam et al. 2007) suggests a startling genomic complication in this then-called primitive group, which is the evolutionary sis of the lineage containing all the bilaterians. This anemone demonstrates the presence of vertebrate-like introns and a factor-linkage pattern also quite similar to that of vertebrates. Genes involved with jail cell adhesion, cell signaling, and synaptic transmission are already present in the anemone, suggesting that the genome was already complex and was modified in various means in descendants. Surprisingly, stronger similarities are seen between anemones and vertebrates than between anemones and flies, suggesting stronger modifications in the lineages containing the latter. But the presence of genomic complexity, right at the dawn of bilaterian animal life, is inescapable. Even deeper in the evolutionary tree we observe Trichoplax, which is mayhap the simplest of gratis-living multicellular metazoans and probable in a sister grouping of the combined group of Cnidarians and Bilaterians. Its genome is also complex and its genome reveals a large array of transcription factor genes and signaling pathway genes plant in more derived bilaterians such as vertebrates, where they are employed to run the many complexities of a cellularly diverse organism (Srivastava et al. 2008). Even so, some increases in complexity, such every bit duplications of Hox genes, are apparent in the rise of the bilaterian line (Martinez et al. 1998).

We can quickly become overboard past accepting the continuance of factor part over such broad sweeps of evolutionary time and taxonomic breadth. Some of the genes thought to be key and constant in the mainstream of development, such as inductive-posterior determination, are employed for a variety of functions. Thus has arisen the concept of a developmental gene tool kit, whose elements may be recruited for many disparate functions in very different cell types. Stages of development, and fifty-fifty fundamental features such as segmentation in unlike groups, may be accomplished by different genes, which are themselves retained over the history of the bilaterians (Grenier et al. 1997). Many of the genes have been in the metazoans since earlier the dawn of the bilateria, which makes information technology difficult to either exclude or ostend the hypothesis that performance bilateria might have evolved far before the Cambrian.

We therefore cannot exclude the hypothesis that bilaterian animals with complex morphology existed earlier the Cambrian explosion merely somehow were not preserved. We have dealt in a higher place with the preservation issue, which is still murky, but nosotros cannot overlook the fact that a treasure trove of Precambrian bilaterian fossils has yet to be found despite much searching.

More exploration into this subject suggests that we still cannot make definitive conclusions almost the meaning of action of ancient genes, although some encouraging progress is existence made. To brand progress, we demand (a) a good fossil record, with preserved characters that can be linked to specific genes, and (b) an understanding of the construction and working of the part of the genome that specifies the traits in question. Bottjer and colleagues (2006), for instance, have succeeded in linking the traits of Cambrian echinoderms to genes involved in biomineralization in a living sea urchin.

We tin illustrate the problem of genome complexity and ancient gene action with the striking universality of the Pax-6 gene in protostome and deuterostome Bilateria. One might conclude that Pax-half dozen is a master eye cistron, especially after learning that the cistron can exist transplanted from a mouse or squid into a fly, resulting in the stimulation of ectopic eyes on diverse inappropriate locations of the fly'south body (Tomarev et al. 1997). This finding is peculiar, given the universal conventionalities (before the discovery of Pax-6, that is) that eyes evolved independently into the many functional forms we run across, sometimes as the issue of independent evolutionary convergence of like and quite detailed structures (e.one thousand., Salvini-Plawen and Mayr 1977).

What explains this patently incongruous result? Developmental relationships testify an association between Pax-6 and anterior neural determination. It may well be that this gene was crucial in an ancestral form's detection of light, but inevitably became linked to the instigation of eye development in all subsequent episodes in heart evolution. Thus, Pax-half-dozen is an inevitable and stable component of eye development, but is certainly not the determinant of the specific form of the many and independent episodes of evolution that led to eye spots, mirrors, chemical compound eyes, and camera eyes in every bit many equally sixty or so evolutionary events.

Also, invertebrate and vertebrate eyes appeared to have fundamentally different embryological origins, transduction mechanisms, and cellular structure. But the polychaete Platynereis dumerilii and other invertebrates accept been found to accept circumstantial vertebrate and invertebrate eye cellular types, which suggests that the common antecedent of vertebrates and invertebrates had both photoreceptor cell types, but that one type has been mainly employed in optics of each major grouping (Arendt et al. 2004). Photopigments may have evolved merely once with a gene duplication, merely photoreceptor cells types have evolved at least twice, and morphologically distinct eyes evolved multiple times.

In that location is an important lesson here. The genes nowadays for complication may accept been present in ancestral bilaterians, simply the genes that make up one's mind the detailed structures and functions (circulation, vision, etc.) we associate with bilaterian development are not specified. The devil is in the details...and in the fossils that demand to be discovered.

The trigger?

Possible triggers for a Cambrian explosion (table two) include (a) extrinsic changes in climate, paleogeography, and ocean chemical science; (b) evolutionary adaptive innovations that encouraged diversification; (c) intrinsic mechanisms, normally involving genetic conclusion of development or other traits; and (d) feedback loops between any of the starting time three mechanisms, causing a self-propagating explosion.

Table two.

Some possible causes of the Cambrian explosion.

Abundant data suggest possible extrinsic triggers for a Cambrian explosion. Leading upwards to the events of most 520 Ma was a flow of about 150 million years of continental breakdown, followed by collisions that occurred in the Early Cambrian. Strontium isotopic evidence (Nicholas 1996) suggests a major increase in terrestrial weathering, which may reflect increased nutrient inputs during this fourth dimension. An increase in oxygen around 600 Ma (Canfield et al. 2007) might have immune the existence of larger-bodied agile animals, especially those with calcium carbonate skeletons. Finally, the aftermath of a world completely covered by ice, followed by strong global warming (Hoffman et al. 1998), might have included an oxygenated body of water that permitted or fifty-fifty stimulated a radiation of bilaterian animal life. But this episode ended nigh 600 Ma, many millions of years before the Cambrian. Why the long lag time before the and so-called explosion? The effect of response fourth dimension is probably the single almost difficult trouble for researchers in macroevolution. There is no theory to tell u.s.a. how fast a major evolutionary alter can occur or how much change should occur, given a predefined prepare of environmental and biological circumstances.

Ane of the obvious difficulties hither is that we are searching underneath the Cambrian lamppost for the keys to an explosion of beast life. If we searched for a Precambrian time of origin, we might observe equally tantalizing explanations. For example, a recent long-term estimate of oceanic temperature shows a driblet at about 1200 Ma from likely limiting high temperatures (approximately seventy degrees Celsius [°C]) to temperatures of 30°C, resembling those of our modernistic body of water (Robert and Chaussidon 2006). This idea is controversial, but it nicely fits some of the Precambrian estimates for the rising of animal life. It might even unify the search for ancient causes with the search for current mechanisms that may limit organismal physiological performance nether global warming.

Extrinsic biological factors might also take been instrumental in selection for diverse lifestyles and morphologies. Spectacular predators existed even in the Lower Cambrian, and predators might have stimulated the evolution of various morphological, chemical, and behavioral defence force mechanisms (Stanley 1976).

Intrinsic factors involve the appearance of a biological innovation whose presence permits a vastly increased potential for diversification. We take discussed above the proposition that Hox genes might have produced a novel developmental mechanism that permitted diversification. The ancient origin of such genes, and even their number, argues against this explanation. Still, given the profound changes occurring at this before phase of evolution, major genetic changes might have been focused on ane part of the genome involved in major developmental shifts. Perhaps the organization of Hox genetic determination differed early in bilaterian evolution, which allowed for profound shifts (Arthur 2000). Gene duplications, even genome duplications, might accept characterized Cambrian bilaterians, thus creating a major opportunity for genetic functional deviation (Lundin 1999). Predetermined biases in cistron-phenotype organization may too have guided evolutionary direction (West-Eberhard 1989). Unfortunately, there is no reason to exclude a host of other genes and developmental mechanisms.

Many other biological innovations have been related to the Cambrian explosion. For case, the evolution of a more highly organized nervous organization might have permitted a broad range of new behaviors and functional connections among body parts, resulting in a broad range of complex inter-species interactions. Because so many of the groups appearing in the Cambrian had an plainly well-organized nervous system, we tin can only speculate on the order of cause and effect, since whatsoever radiation would have produced an assortment of species with complex nervous systems. In any result, all bilaterians share the potential for such a complex nervous system, which might be realized by the necessity of ecology. Arguments for other possible intrinsic factors, such as the appearance of irised color (Parker 1998), accept the opposite drawback, as few groups would have had such features. Could they have precipitated the whole explosion?

A specially intriguing innovation is the power of planktonic animal creatures to settle on the lesser and prefer a more complex benthic lifestyle, replete with features mutual in larger animals, such every bit skeletons and large-scale circulatory systems. Two different types of planktonic larvae are speculated to be the ancestors of the protostomes and deuterostomes, and the benthic descendants would be the germ of the Cambrian explosion. The limited ability of larval cells to elaborate morphologies might take been changed dramatically past the advent of set-aside cells, which resided in the larva merely were used to specify the morphologies characteristic of the larger, more complex benthic developed life stages (Davidson et al. 1995). The latter hypothesis, however, flies in the face of facts: important groups such as arthropods show no strong evidence of such set-aside cells, and other groups take limited evidence for such dramatic changes during metamorphosis from larva to developed (e.g., many annelids). The about bequeathed known group of bilaterians, the acoels, lacks the indirect development expected when set-aside cells exist (Ruiz-Trillo et al. 1999).

Feedback loops accept been suggested between weathering and the development of early life, and betwixt changes in primary productivity and an explosion of life. The increase in weathering might have been profoundly enhanced past the early development of biomineralization, which might have further increased weathering and perhaps nutrient inputs, thus triggering an evolutionary explosion (von Bloh et al. 2003). These events might be linked to a known major turnover in preserved fossil phytoplankton in the Early Cambrian (Butterfield 1997). Phytoplankton would have been consumed by zooplankton, which would have directly provided fecal pellets for consign to the bottom, and uneaten phytoplankton would have sunk to the bottom, with both types of plankton providing a trophic stimulus for benthic animals. Although interesting, this connectedness is entirely speculative and unbounded by any unique data (east.1000., the carbon isotope changes mentioned above).

A path for the hereafter?

Will molecular clocks pave the way? At present, molecular evidence points to a Precambrian divergence for the bilaterian fauna phyla, but the pointer is rather shaky. Nosotros tin can but promise that improve prove will sally from the big-scale sequencing nether way. At present, it is off-white to say that the assumptions behind the methods announced to strongly affect the results, which should heighten skepticism well-nigh the power of molecular clocks to resolve the question of the timing of the divergence of the Bilateria.

Will fossils pave the way? Just new discoveries, combined with sensible analysis of the status of morphological characters, could allow paleontologists to revise the currently robust result that no obvious bilaterian torso plans can be institute in rocks older than the Cambrian. The sudden appearance of such a complex biota in the Early Cambrian argues that at that place may be a missing and probably rich ancestral biota earlier than the Cambrian. These fossils, given the likely depression oxygen of Precambrian oceans, will be small-scale and unmineralized. Such a discovery will certainly not undermine the clear diversification of a large assortment of large-bodied forms in the Cambrian. In that location was an explosion of some sort, but its character needs to be better understood. But only ane Orsten-like Precambrian discovery might unleash a wealth of Lilliputian morphological diverseness that would crack the idea of a Cambrian phylogenetic explosion wide open.

Will gene analysis pave the style? Information technology is painfully apparent that our understanding of the molecular genetic determination of traits is too young to sympathise the meaning of the presence or absenteeism of specific gene complexes. We have excellent evidence of the presence of genes that set up of import developmental processes in motion (east.chiliad., Hox genes, Pax-six factor), but we know piffling of the structure of the downstream genes or of other genes that determine crucial structures of macroevolutionary significance.

Do scientists know enough to understand the step of torso programme development? If the fossil record tells u.s. annihilation, it is that evolution is dominated by radiations of form and genetic structure that are discontinuous in time. Regrettably, we know besides little yet to devise an evolutionary model that would predict the pace of evolution on the level of the major trunk plans of the bilaterian phyla. The few examples discovered and then far of molecular underpinnings of extant morphological polymorphism are enticing (e.g., Colosimo et al. 2005), only far too incomplete to permit us to understand the molecular and genetic ground of major morphological transitions, or of the footstep of evolution for the major body plans. We too must wonder if analyses of extant variation and modify can duplicate the construction of the genome and the genes responsible for the initial fantastic radiation. Equally discouraging is the lack of evidence of definitive geological-climatological events causing evolutionary radiations. We have many potential culprits, merely few definitive causes.

Acknowledgments

This article developed from a symposium cosponsored by the American Institute of Biological Sciences, the National Association of Biology Teachers, and the National Evolutionary Synthesis Center in October 2006. I thank Marvalee Wake and 3 anonymous reviewers for helpful comments. I apologize to my colleagues for using my book as a citation to avert exceeding the reference number limit by too much. This is contribution 1176 from the Program in Ecology and Evolution of Stony Beck University.

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Author notes

Jeffrey S. Levinton (e-post: levinton@life.bio.sunysb.edu) is with the Section of Ecology and Development at Stony Brook Academy in Stony Brook, New York. He is a marine ecologist with broad interests in functional and customs ecology, the evolution of aquatic populations, and macroevolution.

Source: https://academic.oup.com/bioscience/article/58/9/855/251078

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