3. Life Finds a Way

Richard Dawkins has dubbed evolution “The Greatest Show on Earth”. Truly, the origin of life, much of which is still unclear, and the advancement of it with the most surprising modifications is too vast a concept to fathom. Indeed, even the very ‘definition’ of life is a much debated topic among scientists and philosophers. But it is generally agreed upon that living things consume materials and energy, display growth, and reproduce.

But how did it come to be as it is? ‘Life’ on earth began 3.8 billion years ago and has undergone constant modifications and alterations, sometimes for better, sometimes for worse. Life forms that were equipped to go along with the environment survived, others perished. And over this vast range of time the environment underwent a lot of changes forcing the organisms to change accordingly for survival. In the game of survival, staying ‘fit’ is the mantra.

Before we follow the path of life, let us quickly go through some key concepts which enable the scientists to determine the course of evolution and to understand the relations between the diverse biosphere of the earth. This will come in particularly handy when we talk about dinosaurs.

When an organism evolves from another with new features to suit its environment, it does not mean that it has severed all ties from its ancestors. It has genetic connections with its ancestors. And this is where the principle of homology hits the mark, which basically says that the genetic relationship among organisms characterise genetic relationship among their parts. Such anatomical structures are called homologs and they can be traced back to a parent anatomical feature in a common ancestor.

                                     Fig 3.1- Homologous anatomical structures

 
Figure 3.1 makes the structural similarity clear. From this we conclude that the similarity between a seal’s flipper and a human hand is not an independent occurrence but an indication of the fact that they share a common ancestor somewhere back in time. In other words, they are just two different branches which can be traced back to a single node in the tree of life. They have their forelimbs modified according to their environmental needs. These structurally similar organs may serve different purposes in different organisms, as compared to analogs which serve the same purpose but are structurally different.

                                                 
                                            Fig 3.2- Analogous organs

Now, as we have already learned that fossils are very rare, it is understandably much rarer that a fossil found is an ancestor of some other fossil. But what we may get from the fossil is some features that the ancestors might have possessed. Thus we get an approximation to how the ancestor was like.

The next important concept is that of phylogeny. It would not take an evolutionary biologist to conclude that a dog and a wolf are more closely related to each other than a dog and a horse. The common sense we are applying is that more specific features shared imply closer relation. Phylogenetic systematics looks for unique features to infer about relationships between organisms. This is done using the knowledge of hierarchy.

All features of living organisms are hierarchically distributed. To take an example, all organisms with feathers form a subset of all creatures with a spinal cord, which is a subset of all living beings. So we recognise birds as creatures with feathers. So, it is easy to tell that a sparrow and a crow are more closely related than a crow and a bat. But if we were to determine this same relationship among an eagle, a hawk and a duck, the feature of having feathers would be useless and we would have to go down deeper in the hierarchical ranking looking for other features to conclude that the eagle and the hawk are closer. Scientists look at these features and carefully set up the hierarchy and on the basis of that try to draw a picture of the evolution. This process is often quite difficult with the volume of features to deal with, and a wrong hierarchical set up would give an erroneous picture.

Now that we have a high level idea of how scientists infer about evolution, let’s see what they have inferred about evolution.

I have mentioned earlier that the biosphere is 3.8 billion years old. However, we will omit discussions about molecular life as it requires considerable training in relevant disciplines, making it somewhat beyond the scope of our discussion. We can only mention that we eventually had bacteria and other single-celled organisms, which gradually changed into multi-cellular organisms. This is the Precambrian period, a time also known as Cryptozoic (“hidden life”). About 590 million years ago (the unit ‘million years ago’ is abbreviated as Ma), the Precambrian period came to end and is the start of the Cambrian period. It is from this time proper fossil records are found. And the turn evolution took in this period is called “Cambrian explosion”.

Some organisms at this time developed the ability to absorb mineral calcite from the seawater and form chitin, giving rise to the first shelled creatures. These creatures now had an advantage over its predators. Consequently, some predators developed new structures to even out the odd. And the evolutionary arm race had begun.

The early life forms were all marine. The Cambrian oceans were full of diverse creatures with unusual features most of which did not survive towards the end of the Cambrian period (505 Ma). Only a few did. These creatures were the pioneers of the stable evolutionary lines. There was the Trilobite, an early arthropod.

                                                Fig 3.3- Trilobite fossil

Also, there were early chordates, creatures with spinal cords, like the Pikaia. The time from the Cambrian to present day is termed phanerozoic (“obvious life”).

   Fig 3.4- Artist's impression of Pikaia

The periods that followed the Cambrian time were the Ordovician and Silurian times (505-408 Ma), the time when first vertebrates arrived. These were fish, but they had not formed a jaw yet, they had suckers to collect food from the ocean floor. Then gradually they developed well-structured skeletons made of cartilage, very much like today's sharks. Later formed bones and scales, very similar to the fish we know today. This was the period Devonian (408-360 Ma), “the Age of Fish”. 

                                    Fig 3.5- Dunkleosteus, armoured fish            

Something very significant occurred in this time. Plants, the pioneers of life on land, began colonising the landscapes. They could flourish due to the absence of land herbivores. And they made lands inhabitable for other beings by increasing the oxygen content in the air. Arthropods infested those forests and then followed the fish. Some fish developed lungs to be able to breathe out of water, some even developed muscular fins to be able to drag themselves and crawl on land as well. It is suggested that this ocean to land journey occurred in temptation of new food source. A rival theory says that it was a measure to avoid the predator infested waters. Either way, life began on land.

The end of the Devonian times witnessed the first amphibians, creatures that spend their time on land, but had to go back to the waters to breed. Such a creature was the Ichthhyostega (“fish roof”). It had the head and tail of a fish, but had developed four eight-toed 'legs'. 

                                              Fig 3.6- Devonian Life 

 

The next big evolutionary achievement was the ability to breed on land. A group of creatures called the amniotes (the membrane that holds the young in the egg is called amnion) laid hard-shelled eggs that had no need of water. The migration from sea to land was now complete. This was the Carboniferous period. 

                                      Fig 3.7 - Westlothiana, an amniote; 

                                                      fossil and restoration

 

Now let us look at another type of creatures that evolved during the late Carboniferous. These were the reptiles. They can be classified on the basis of arrangement of holes in the skull.

The anapsids had no hole in the skull other than the eye-socket and the nasal holes. Their contemporary relative are turtles and tortoises.

The synapsids had a hole at each side of the skull. They were the mammal-like reptiles. These holes in the skull made the skull lighter and the jaws more dynamic for efficient biting. The fittest creatures were in making.

           Fig 3.8- Skull comparison: A-anapsid, B-synopsid, C-diapsid

   

The diapsids had two such pairs of holes behind the eye-sockets. Their modern day relatives are snakes, lizards, crocodiles and birds. A group of such diapsids, called the archosaurs (“ruling lizard”) made their way into the Mesozoic era to establish themselves as the apex creatures. It was from these archosaurs, during the middle Triassic period, evolved the dinosaurs.