What is evolution about? Why are there different species, rather than just one (or a few) highly variable species? Is there a close correspondence between the ecological “spaces” that organisms can fit and the adaptations … represented by morphology, for instance … of the species that do exist? Can you imagine a different world where instead of having 10,000 species of birds there is only one bird that is highly adaptable in its behavior, able to change diets or nesting patterns as needed to fit to any given ecological niche?
These questions are of the type that require the study of living forms and the fossil record at a “macro” level (which does not necessarily imply that we would be studying “macroevolution” … just evolution in a more general sense at a large scale). And these are the kinds of questions that addressed, to varying degrees, by a paper just out in Science by Shen et al called “The Avalon Explosion: Evolution of Ediacara Morphospace.”
You probably already know that there was an “event” called the Cambrian Explosion about 530 million years ago. For a long time this was thought to have been the origin of multicellular (“complex”) life forms. It was characterized by the seemingly sudden appearance of a large number of diverse forms, many traceable as ancestors of more recent and living forms. In the days when there was no clear evidence of earlier fossils, it was postulated that this was the beginning of multicellular life, and that this origin was explosive in its timing and effects.
This conception also resulted in the dubbing of earlier strata a “pre-Cambrian,” a term linked to the Cambrian as the origin of multicellular forms. In other words, we could divide evolutionary time into two periods, the time when there were only single celled life forms (the pre=Cambrian) and the time after multicellular life forms suddenly appeared (the Cambrian and onwards to the present).
This linkage between terms and meaning is unfortunate, because it places all pre-Cambrian fossils in a special category. Perhaps Cambrian and subsequent periods are “special” in that many Cambrian forms are ancestral to existing forms, while earlier multicellular fossils are difficult or impossible to link to later life forms. Nonetheless, if you went back in time to the Cambrian as an actual Cambrian life form with a preternatural interest in and aptitude for paleontology, you might have a different view. You might see less discontinuity in your own tens-of-millions year long history.
Getting back to the questions raised above: We see around us today that a diversity of species has filled a diversity of ecological niches. These niches can be defined coarsely by fundamental gradients or transitions across axes such as equatorial to polar, terrestrial vs. aquatic, rainfall regimes, etc. To some extent these gradients or transitions determine the adaptations we see, but it is also true that certain adaptations seem to show up nearly everywhere. There are no terrestrial environments that lack flying vertebrates, always including some birds, sometimes bats, and occasionally other forms. A large mouthed, large bodied ambush predatory fish seems to exists in every non-riverine aquatic environment. And so on. Multicellular life today seems to have a pattern, even if a very simplistically defined pattern, in that the big picture is the same everywhere (all but a few terrestrial habitats have grazer/browsers) with internal patterning imprinted on this larger scale pattern. (Grazers are more common in grasslands, browsers in forests).
The new paper by Shen et al. examines the ‘species’ diversity and overall morphological range of adaptation (measured here as “morphospace”) for three faunas that predate the Cambrian explostion: Avalon (575-564 million years ago); White Sea (560-550 million years ago) and Nama (550 to 542 million years ago).
The researchers characterize the faunas (collections of animals in this case represented as fossils) with respect to their morphological diversity and the number of different forms (species, but we are really working at the level of genus in this case). I don’t want to go into the details of how “morphospace” is measured, but I can give you a good idea of how to understand it. Imagine the fauna of your own neighborhood, which includes some larger herbivores (deer, antelope, or cattle) some terrestrial predators (bobcats, canids, etc) flying herbivores and carnivores (sparrows and sparrow hawks?), fossorial rodents and insectivores (mice and shrews) and so on. If you measure the presence/absence and possibly various metrics of the body parts of these organisms, you will find that that there is a certain range, or breadth, of diversity … there are things with long legs, things with wings, things with teeth that can be used for digging, other things with sharp teeth for killing, and so on. You will also notice that these features are distributed among a certain number of types of organisms. Now, imagine removing from this habitat all things that fly. Now, there are no more wings, no more perching feet, no more structurally light-weight skeletal elements, etc. in the assemblages. The morphospace is smaller. Alternatively, you could remove some but not all of one “kind” of organism. Remove all bats and all but two species of birds. You have the same morphospace (roughly) but distributed differently among the different species.
Most people in the business would say that morphospace characteristics do not change much these days; the morphospace qualities of the Holocene (the Present) is similar to the Miocene, and so on. The extent that the morphospace characteristics have changed since the end of the Permian is probably minimal. Yet, there are major transitions. Prior to the End Permian extinction a very large percentage of aquatic animals were sessile … like plants, stuck to something … while there were plants floating around free. Subsequently, that situation is largely reversed. Obviously, a major transition is from the era when there were no multicellular forms to the period being discussed here and subsequently, when there are multicellular forms.
So the researchers are asking two questions along a number of dimensions. First, are there changes in species diversity, and separately, are there changes in morphospace, across these differently aged faunas? Second, are there variations within any one or more of these periods in species diversity or morphospace that can be explained by large scale transitions or gradients such as depth in the sea at which the faunal lived (I should mention that these are all marine organisms) or distance form the equator?
This a difficult question to answer. The various fossil sites that make up each of these faunas are from a range of environments, and obviously, time periods. The assemblages differ in their taphonomic qualities. In other words, they differ in the nature of preservation and in potential biases in what is preserved and what is not preserved. And given that we are talking about a 32 million year long time period, the samples are very sparse. How many species actually exist, arise and go extinct, over any 32 million year time period? More than there are currently fossils to represent from this distant era.
Shen et al take all of these things into consideration and come up with a somewhat guarded answer: No.
The number of genera represented in each fossil collection varies but not to the extent that requires any explanation other than sampling and taphonomic effects. There are variations in morphospace but, again, nothing that requires an explanation other than a reasonable degree of variation buffeted by the ravages of time.
Interestingly, this study of pre-Cambrian evolutionary dynamics fails to answer the original question: What was going on with the Cambrian Explosion? Did the pre-Cambrian involve a ramping up of diversity that suddenly becomes apparent in the Cambrian, with the “explosion” being a sampling effect? Did pre-Cambrian life forms represent a different evolutionary pattern than seen in the Cambrian?
The answer is, well, the Cambrian explosion was precede by three previous explosion sof a similar nature. Now, we need to explain the Avalon Explosion. Back to square one.
What might have led to the rapid morphospace expansion in the Avalon assemblage, and what might have constrained the Ediacara morphospace from further expansion or shift in the subsequent White Sea and Nama assemblages? We consider a long, undocumented period of Ediacara history before the Avalon assemblage to be unlikely. The rapid increase of morphospace at the beginning of Ediacara evolution parallels the disparity patterns of the Cambrian explosion [note: "disparity" refers to the degree of morphological difference among a group of species. Diversity = many species, Disparity = many different species] … a rapid evolution of body plans followed by taxonomic diversification within the limits of a predefined morphospace. Various environmental, ecological, and developmental factors have been proposed to explain the rapid evolution of animal body plans during the Cambrian explosion, … In principle, these explanations may also be applied to the Avalon radiation. … Regardless … the marked parallels between the Cambrian and Avalon explosions suggest that the decoupling of taxonomic and morphological evolution is not unique to the Cambrian explosion and that the Avalon explosion represents an independent, failed experiment with an evolutionary pattern similar to that of the Cambrian explosion.
In other words, the main difference between the Avalon Explosion and the Cambrian Explosion is that the latter is still underway.
Shen, Bing, Lin Dong, Shuhai Xiao, Micha Kowalewski. (2008) The Avalon Explosion: Evolution of Ediacara Morphospace. Science 4 January 2008:Vol. 319. no. 5859, pp. 81 – 84. DOI: 10.1126/science.1150279