Hello. My name is Danny Eibye-Jacobsen and I work at the Natural History Museum of Denmark, where I carry out research on annelid worms. In this presentation, I will be talking about the Cambrian Explosion. As you will see, different people mean different things when they talk about the Cambrian Explosion, but as a point of departure we can think of it as the apparently rapid rise of multicellular life in the oceans that took place during the beginning of the Cambrian Period. This was the very beginning of the so-called Palaeozoic Era, immediately following the Precambrian Era. The last period of the Precambrian was the Ediacaran and the border between the Ediacaran and the Cambrian is now set at about 541 million years ago. The Cambrian Explosion is of particular interest for two reasons. First of all, it is the story of how most of the animal groups that we are familiar with today arose and came to dominate the world. Secondly, the apparent speed of the changes that took place in animal life during the Cambrian Explosion is remarkable and raises questions about the evolutionary mechanisms behind them. This presentation has three basic parts; the first one will be in this video and the other two in the following one. First, I will try to reach a concise definition of the Cambrian Explosion. Then we will go through the large number of factors that may or may not have contributed to or even triggered the explosion. Finally, I will try to put all these pieces together from the point of view of the ecosystem that the Cambrian organisms were a part of. Thus, what exactly was the Cambrian Explosion, what caused it and what did it lead to? I would like to start with an historical perspective – with something that has been called Darwin's Dilemma. When Charles Darwin published the first version of The Origin of Species in 1859, he was brutally honest about what he perceived to be weaknesses in his theory of evolution. One of the most serious of them referred to what is now popularly called the Cambrian Explosion. The theory of evolution postulates the slow and gradual development of organisms from very simple forms in some distant past to the astounding diversity that we see in the present-day world. Such a scenario placed firm expectations on, and thus predictions about, the fossil record. Unfortunately, the fossil record, as it was known in Darwin's time, told a very different story. The oldest known fossils were trilobites, like this Eofallotaspis from Morocco, closely followed by brachiopods and molluscs. But the trilobites appeared to have arisen out of nothing, as all the older geological layers that had been studied at that time were barren. Trilobites were understood to be complicated animals, nothing at all like the "primitive" forms that Darwin would have expected. This was Darwin's Dilemma and it is also the original and most simple concept of the Cambrian Explosion: that complicated animals suddenly appeared in the fossil record without the preceding layers containing any less complicated animals. At the time Darwin had no choice but to assume that there must have been a vast period of time prior to the trilobites of the Cambrian Period in which life was abundant in the primeval seas without leaving any traces of its existence – a "lost world". Remember that another weakness of the original theory of evolution was that Darwin did not understand the actual mechanism of heredity, since he obviously had no knowledge of DNA or genes. In a similar fashion, the solution to at least most of Darwin's Dilemma lies in discoveries that were made after his time. I am going to go rather quickly through the most important of these discoveries as they pertain to multicellular animals to give you an overview. Don't worry – I will talk about most of them in greater detail later. First of all, palaeontologists have found significant portions of Darwin's "lost world". The first known trilobite with a hard skeleton is Eofallotaspis, which first appeared about 530 million years ago during Stage 2 of the Cambrian Period. A large number of finds have documented that the first trilobites occurred together with a large number of animals that were only known by their tiny shells, tubes, spicules or ossicles. The producers of these so-called "small shelly faunas" dominated the oceans from the beginning of the Cambrian 541 million years ago to the beginning of Stage 3 of the Cambrian Period – about 521 million years ago. Furthermore, metazoan life is now also known from the last Precambrian period, the Ediacaran. The period from 575 million years ago to the Precambrian-Cambrian boundary was dominated by the enigmatic Ediacaran faunas. Of the approximately 100 known Ediacaran species, almost all of which appear to have been soft-bodied, as few as 4 or 5 may have been bilaterian metazoans belonging to phylum-level clades that exist today. This field of study is very contentious, with some workers having much higher estimates. The morphologies of the fossils of that period and especially the trace fossils that they left behind clearly demonstrate that organisms of that time were "more simple" than those of the Cambrian, and this is just as Darwin would have predicted. The famous Doushantuo fossilized embryos, which are now dated to about 555 million years ago, included forms that appear to have been bilaterian metazoans as well. And if we go back even further, the oldest undisputed metazoan fossils are a series of circular impressions from the Twitya Formation in the Mackenzie Mountains of northwest Canada. They are about 610 million years old. The point of all this is just to demonstrate that we now know that many different types of animals actually existed and were present in the seas before the Cambrian trilobites. Continuing our overview of knowledge that we have acquired since Darwin's time, life during the Cambrian itself is now understood in much greater detail. Possibly the most significant discoveries in terms of our understanding of what life was like during the Cambrian are the so-called exceptional preservation faunas. They are unique because, in addition to the typical hard skeletons and shells of normal fossil beds, they include preserved soft parts of animals. The most famous of these exceptional preservation faunas or "Lagerstätten" as they are also called, is the Middle Cambrian Burgess Shale of British Columbia in Canada, about 505 million year old. The Chengjiang or Maotianshan fauna of South China is somewhat older, about 520 million years old, and it is now almost as famous. Other important examples of exceptional preservation are the Sirius Passet fauna of northern Greenland and the Emu Bay Shale of South Australia, both about 515 million years old, as well as the phosphatized Orsten fossils of Sweden from the Late Cambrian, probably about 495 million years old. It should be noted that the Ediacaran faunas that I talked about earlier are definitely also exceptional in the form of preservation. Paralleling this vastly increased knowledge of the metazoan animals that existed during the Ediacaran and the Cambrian, we now have a much better understanding of the physical environment of those times. Several changes in the environment are believed to have either facilitated or have been caused by the diversification of life during these periods. One of the possible triggers of the Cambrian Explosion was the series of global glaciations that took place during the Ediacaran Period and the Cryogenian Period before it. Other important factors were the significant tectonic activity and continental drift that characterized the Lower Cambrian and especially the steady increase in atmospheric oxygen that occurred during the Ediacaran and Cambrian. All these factors led to significant changes in ocean chemistry, especially phosphate content, and their combined effect is mirrored in changes in the relative occurrence of carbon isotopes that have been measured in the sediments of those times. Although it is often difficult to separate cause from effect when evaluating these physical factors, it is clear that the Late Ediacaran and the Early Cambrian were periods during which profound changes were taking place in the physical environment. For the final element of this overview, we must return to genetics. Our understanding of the remarkable developments that took place in the animals themselves during the Ediacaran and in the Cambrian has recently been increased considerably by molecular studies. The genomes of recent animals have allowed us to estimate molecular phylogenies to supplement those derived from morphological and anatomic studies. Molecular studies have given us the concept of the molecular clock, which allows us to make predictions about when significant splits in metazoan phylogeny took place. Specific genes, especially the embryonic patterning genes like the Hox genes and the so-called housekeeping genes, are of great importance in understanding these events. There are a number of theoretical and technical problems involved in employing molecular clocks to estimate when important splits in metazoan evolution took place. However, the methods that are being used are constantly being refined and the results are starting to converge with those from fossil evidence. Recent estimates lie between 1.067 and 760 million years ago for the rise of Metazoans, the multicellular animals, and between 760 and 640 million years ago for the rise of Bilateria, the bilaterally symmetrical animals. Regardless of the inaccuracies, the main conclusion that we can extract from this is that bilaterian metazoans had almost certainly already existed for tens of millions of years by the time we reach the beginning of the Cambrian. This is basically in agreement with the fossil evidence, although obvious bilaterians are frustratingly rare in finds from the Ediacaran Period. With all this new information, Darwin's Dilemma can be defined more narrowly: at the end of the Ediacaran only a few large metazoans appear to have existed, but by the time of the Chengjiang fauna 21 million years later, metazoan life was extremely diverse and many of the approximately 35 animal phyla that we recognize today had already appeared on the scene. Thus, metazoan life underwent a dramatic change in the course of approximately 21 million years and that was the Cambrian Explosion. It is fair to ask whether something that took 21 million years to play out can reasonably be called an explosion. The term is used here to signifiy that the changes that took place during this period were truly remarkable and significantly greater than anything that had occurred in the Precambrian times. Some would even argue that these changes were greater than and more rapid than anything that has happened since the Cambrian as well. As always in science, increased knowledge creates new questions, some of which I will discuss in greater detail later. For example these five: 1) Probably the most significant biological aspect of the Cambrian Explosion was a dramatic increase in the number of forms that had a hard skeleton, like a shell, a shield, or plates. Such animals obviously had a much greater chance of becoming preserved as fossils than those with a soft body. And this raises the question of whether the Cambrian Explosion, as we observe it in the fossil record, actually was an increase in the diversity of animals during this time or was it primarily just a simple increase in the number of fossils that have been preserved? 2) Which factors actually caused the Cambrian Explosion and how did they interact? 3) Which major changes took place in the biological environment during the Cambrian Explosion, which is to say: how did the Cambrian ecosystems differ from those of the Ediacaran? 4) Since the precursors or stem forms of several phylum-level clades of bilaterian metazoans must have been present during the late Ediacaran, why haven't they been found? Were they too small to become fossilized or did they occur in other Ediacaran habitats that we just haven't found yet? Or has the known Ediacaran fauna simply been misinterpreted – are they actually the missing bilaterians? And finally 5), was the Cambrian Explosion so exceptional that it cannot even be explained in terms of conventional evolutionary theory? There was a famous controversy between Stephen Jay Gould and Simon Conway Morris during the end of the 20th Century and it was rooted in this question. Another major component of their disagreement concerned the role of contingency or randomness as opposed to predictability or inevitability in the evolution of life during and since the Cambrian. Gould emphasized randomness, Conway Morris emphasized predictability. We will take a closer look at the most important factors involved in the Cambrian Explosion and the ecosystems that existed at that time in the next video.
