The Cambrian Mystery

[Mudhammam]

"In the rise of the metazoans (animals), one would expect that soon after their appearance in the fossil record they would consist of a series of rather similar orders that would become increasingly more dissimilar to each other in the course of time. Yet the facts are astonishingly different from this assumption! When the metazoans appeared as fossils about 550 million years ago (admittedly they must have already existed for ca. 200 million years), they included four to seven bizarre body plans that soon became extinct. All the other Cambrian phyla survived, and what is quite unexpected, without a major revolution of the basic body plan. If we look at individual phyla, the same situation is encountered. The living classes of arthropods are already found in the Cambrian with the same body plans. But again there are a handful of strange types of arthropods in the Cambrian that do not exist today. I agree with those who conclude from this evidence that the variety of realized body plans was greater in the Cambrian than it is now. Furthermore, no fundamentally new body plan has originated in the 500 million years since the Cambrian." - Ernst Mayr, What Evolution Is, pg. 209

Thoughts? Ideas? Have any good theories been put forth to account for this?

[Mister Agenda]

It was quite a mystery in the late 1800s when the Burgess Shale fossils were first discovered, but more than a century of new finds make it clear that the 'explosion' happened over the course of at least 30 million years. A lot can happen in 30 million years. There's nothing here that's a problem for the theory of evolution.

[Mudhammam]

It was quite a mystery in the late 1800s when the Burgess Shale fossils were first discovered, but more than a century of new finds make it clear that the 'explosion' happened over the course of at least 30 million years. A lot can happen in 30 million years. There's nothing here that's a problem for the theory of evolution.

I know it has something to do with regulatory genes but how come new body plans have not come about since the Cambrian explosion? What environmental conditions account for the sudden appearance and diversity of skeletal body plans at ONLY that time? Sure there may have been more oxygen in the atmosphere but how come fundamental body plans haven't diversified since then as we might expect?

[Mister Agenda]

It was quite a mystery in the late 1800s when the Burgess Shale fossils were first discovered, but more than a century of new finds make it clear that the 'explosion' happened over the course of at least 30 million years. A lot can happen in 30 million years. There's nothing here that's a problem for the theory of evolution.

I know it has something to do with regulatory genes but how come new body plans have not come about since the Cambrian explosion? What environmental conditions account for the sudden appearance and diversity of skeletal body plans at ONLY that time? Sure there may have been more oxygen in the atmosphere but how come fundamental body plans haven't diversified since then as we might expect?

First, you would have to establish that we would expect them to. What new body plans should we have expected to appear that are missing?

Natural selection is conservative, it preserves what works for as long as it works better than the alternatives.

[Anomalocaris]

"In the rise of the metazoans (animals), one would expect that soon after their appearance in the fossil record they would consist of a series of rather similar orders that would become increasingly more dissimilar to each other in the course of time. Yet the facts are astonishingly different from this assumption! When the metazoans appeared as fossils about 550 million years ago (admittedly they must have already existed for ca. 200 million years), they included four to seven bizarre body plans that soon became extinct. All the other Cambrian phyla survived, and what is quite unexpected, without a major revolution of the basic body plan. If we look at individual phyla, the same situation is encountered. The living classes of arthropods are already found in the Cambrian with the same body plans. But again there are a handful of strange types of arthropods in the Cambrian that do not exist today. I agree with those who conclude from this evidence that the variety of realized body plans was greater in the Cambrian than it is now. Furthermore, no fundamentally new body plan has originated in the 500 million years since the Cambrian." - Ernst Mayr, What Evolution Is, pg. 209

Thoughts? Ideas? Have any good theories been put forth to account for this?

I think there are two question in your post:

Q1. What accounts for the sudden fossil appearence of metazoans.

Q2. What accounts for the fact that instead of start with a small number of simple body plans (phylums) and gradually diversify into larger numbers of body plans (phylums) over the entire fossil history of metazoans, metazoans instead appear to burst upon the scene with a very large number of body plans, but only a few survive a long time afterwards?

A1. This perception is increasingly attributed to fossilization bias (conditions prior to cambrian didn't favor fossilization, thus while fossils appears to abruptly increase at number of type at beginning of cambrian, animal life had really had a long, but poorly documented history on earth prior to cambrian). There are increasing amounts of fossil evidence since late 1980s to show complex animal life most probably existed not for millions, but hundreds of millions of years prior to beginning of cambrian. They left sparse fossil evidence, but they did leave some fossil evidence.


A2. The notion that there were many more basic body plans in Cambians fossils than in the modern world was popular in late 1980s and early 1990s, notably championed by Harvard Paleontologist Steven Gould.
But that position has been met with increasing skeptism since the 1980s. Many seemingly unusual fossils judged to lie outside of any currently existent body plans by during the 1980s by Gould are now recognized to either fit within existent phylums, or are very close relatives of currently existent phylums, and definition of existent phylum can be slightly and reasonably modified to acommodate them. All modern phylums still do appear to have already existed at the beginning of Cambrain, but this does away with the notion that many more phylums had existed in Cambrian than now. More recent analysis also seem to suggest diversity in fact remained low but gradually increasing through cambrian, and only achieve levels comparable to today after the end of cambrian. Also, genetic evidence suggests all the modern phylums didn't split off from each other more or less at the same around beginning of cambrian, but instead over a period of 300 million years or so before cambrian.

So to wrap up - Although the case isn't closed, it does seem there were:

1. No real explosion, only the appearence of an explosion.

2. A large number of phylums didn't all explode into existence at the same time and then get whittled down. Phylums arose slowly and it appears once a phylum is established, nothing yet happened on earth can cause a whole phylum to go extinct.

[Mudhammam]

First, you would have to establish that we would expect them to. What new body plans should we have expected to appear that are missing?

Natural selection is conservative, it preserves what works for as long as it works better than the alternatives.

Is there anything that accounts for the abundance of diverse phyla at that time, said to number around 60 (while only 35 different animal phyla exist today), or was natural selection simply less conservative in the pre-Cambrian era?

[Anomalocaris]

First, you would have to establish that we would expect them to. What new body plans should we have expected to appear that are missing?

Natural selection is conservative, it preserves what works for as long as it works better than the alternatives.

Is there anything that accounts for the abundance of diverse phyla at that time, said to number around 60 (while only 35 different animal phyla exist today), or was natural selection simply less conservative in the pre-Cambrian era?

It is no longer believed that there were so many phyla during that time. All forms from Cambrian appeal to fit within modern Phyla.

[Mudhammam]

Is there anything that accounts for the abundance of diverse phyla at that time, said to number around 60 (while only 35 different animal phyla exist today), or was natural selection simply less conservative in the pre-Cambrian era?

It is no longer believed that there were so many phyla during that time. All forms from Cambrian appeal to fit within modern Phyla.

Thanks for the help, Chuck. This Mayr book from 2001 seems unfamiliar with those discoveries and modifications. I take it they have only developed within the past 15-20 years? Any good resources you could point me to for further investigation?

[Anomalocaris]

It is no longer believed that there were so many phyla during that time. All forms from Cambrian appeal to fit within modern Phyla.

Thanks for the help, Chuck. This Mayr book from 2001 seems unfamiliar with those discoveries and modifications. I take it they have only developed within the past 15-20 years? Any good resources you could point me to for further investigation?

Actually, It's not entirely accurate to say they only developed within the past 15-20 years. The notion that most or all of known Cambrian fossil belonged within currently existent phyla had in fact been the orthodox interpretation of available fossil evidence amongst paleotonological community all along, and has been since the first major discovery of Cambrian fossils in early 20th century.

The new notion that many of the fossils actually represent previously unknown phyla fundamentally different for currently existent phyla was not based on any new evidence. Instead it is based radical reinterpretation of some of the earliest evidence ever found for Cambrian life. The new view was exciting to younger researchers because it appear to offer the opportunity for a radical change in the direction of the field, and with it many new career opportunities. But it gained relatively little traction with majority of established professional opinion.

Steven Gould, a major champion of the new view, was in addition to being a highly respected paleonotologist, also a world class science writer, science advocate, and polymath. He published beautifully written essays and books to argue his case to the general public. His popular science books made pretty high on New York Times best seller list. He position may thereby have gained visibility and currency with general public somewhat out of proportion to its acceptance amongst mainstream professionals.

The debate amongst professionals became rancorous and very public. An eminent, equally highly respected Cambridge paleotologist whose work Gould cited as a main support for his own position then published a equally well written, but scathing and no holds barred book repudiating Gould's position.

Soon Gould's acolytes and advocates from the rival, orthrodox camp were lobbing flaming books at each other. I have not read Mayr, but i suspect it might have been a shell fired in one of these barrages.

Part of the reason why the debate became so heated was for better part of a century, much of paleotological knowledge of the Cambrian fauna came from a single large haul of fossils discovered in Canada, near Banff national park, in early 20th century, called the Burgess Shale fossils. Burgess Shale, representing one moment in time during Cambrian and one location on earth, was 90% of what was known about Cambrian life. The world inferred from Burgess Shale fossils became known as the burgess fauna.

Since Burgess Shale fossil was all there was, Both sides simply looked at the same Burgess Shale fossils in different ways, and arrived at different conclusions about the burgess fauna, and from these inferred a differnt Cambrian world. Although the orthodox interpretation was never dethroned from its position, neither side could really convince the other.

But in the 1980s and 1990s, additional large hauls of Cambrian fossils were discovered in china and elsewhere. Instead of all from the same place and same moment in time, these new finds allowed pictures of cambrians fauna to be taken at different moments in time, both before and after Burgess Shale, and at widely different types of prehistoric localities. In addition paleotologists also went back to burgess shale and discovered more stuff that had been overlooked nearly a century ago, this allowed even the fossils already discovered at Burgess Shale to be better reinterpreted. These greatly fleshed out what is known about cambrians fauna. Instead of isolated specimens, now it become possible to find relationships between specimens, across time, and across different types of marine environment at different locations on earth. Soon comparison between Burgess Shale fossils and newly discovered fossils from china and elsewhere began to make it increasingly clear that many of Gould camp's interpretation of the structures and the orientations of the creatures found in Burgess Shale fossils were not correct. The interpretation that these fossils represented something radically different from existing fauna was not warranted.

I think Gould camp has clearly lost.

[Mudhammam]

Awesome overview of the issue. Really appreciate it, great read.