MacIver and Lars Schmitz , a paleontologist at the Claremont Colleges, have created mathematical models that explore how the increase in information available to air-dwelling creatures would have manifested itself, over the eons, in an increase in eye size.
MacIver first came up with his hypothesis in while studying the black ghost knifefish of South America — an electric fish that hunts at night by generating electrical currents in the water to sense its environment. MacIver compares the effect to a kind of radar system.
Being something of a polymath, with interests and experience in robotics and mathematics in addition to biology, neuroscience and paleontology, MacIver built a robotic version of the knifefish , complete with an electrosensory system, to study its exotic sensing abilities and its unusually agile movement. When MacIver compared the volume of space in which the knifefish can potentially detect water fleas, one of its favorite prey, with that of a fish that relies on vision to hunt the same prey, he found they were roughly the same.
This was surprising. Because the knifefish must generate electricity to perceive the world — something that requires a lot of energy — he expected it would have a smaller sensory volume for prey compared to that of a vision-centric fish.
At first he thought he had made a simple calculation error. But he soon discovered that the critical factor accounting for the unexpectedly small visual sensory space was the amount that water absorbs and scatters light. In air, light can travel between 25 to kilometers, depending on how much moisture is in the air. Because of this, aquatic creatures rarely gain much evolutionary benefit from an increase in eye size, and they have much to lose.
Eyes are costly in evolutionary terms because they require so much energy to maintain; photoreceptor cells and neurons in the visual areas of the brain need a lot of oxygen to function.
Therefore, any increase in eye size had better yield significant benefits to justify that extra energy. MacIver likens increasing eye size in the water to switching on high beams in the fog in an attempt to see farther ahead.
But once you take eyes out of the water and into air, a larger eye size leads to a proportionate increase in how far you can see. MacIver concluded that eye size would have increased significantly during the water-to-land transition. When he mentioned his insight to the evolutionary biologist Neil Shubin — a member of the team that discovered Tiktaalik roseae , an important transitional fossil from million years ago that had lungs and gills — MacIver was encouraged to learn that paleontologists had noticed an increase in eye size in the fossil record.
MacIver decided to investigate for himself. MacIver had an intriguing hypothesis, but he needed evidence. MacIver and Schmitz first made a careful review of the fossil record to track changes in the size of eye sockets, which would indicate corresponding changes in eyes, since they are proportional to socket size.
The pair collected 59 early tetrapod skulls spanning the water-to-land transition period that were sufficiently intact to allow them to measure both the eye orbit and the length of the skull.
Then they fed those data into a computer model to simulate how eye socket size changed over many generations, so as to gain a sense of the evolutionary genetic drift of that trait.
They found that there was indeed a marked increase in eye size — a tripling, in fact — during the transitional period. The average eye socket size before transition was 13 millimeters, compared to 36 millimeters after.
In addition, these early amphibians were large-bodied animals with strong bodies and prominent ribs - quite different in appearance from modern representatives such as frogs and axolotls. It was originally believed that the tetrapods evolved during periods of drought, when the ability to move between pools would be an advantage.
The animals would also have been able to take advantage of terrestrial prey, such as arthropods. Juvenile animals could avoid predation by the land-based adults by living in shallow water. However, fossil and geological evidence tells us that the early tetrapods lived in lagoons in tropical regions, so that drought was not an issue. They were unlikely to be feeding on land: arthropods are small and fast-moving, unlikely prey for large, sluggish amphibians.
But amphibians that laid their eggs on land, rather than in water, would be at a selective advantage, avoiding predation by aquatic vertebrates such as other amphibians and fish on gametes, eggs and hatchlings. Even today some amphibians e. However, they must still be in a moist environment, and the size of the egg is restricted to less than 1. This is because the egg is dependent on diffusion alone for gas exchange, and means that the embryo must develop rapidly into a food-seeking larval form rather than undergo prolonged development within the egg.
In the Devonian seas, brachiopods had become a dominant invertebrate group, while the fish continued to evolve, with sharks becoming the dominant marine vertebrates. The placoderms and acanthodian fish were quite diverse during the Devonian, but their numbers then dwindled rapidly and both groups became extinct by the end of the Carboniferous period. Lobe-finned fish also peaked in numbers during the Devonian.
Early reptiles and the amniotic egg. One of the greatest evolutionary innovations of the Carboniferous period - million years ago was the amniotic egg , which allowed early reptiles to move away from waterside habitats and colonise dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the embryo inside from drying out, so eggs could be laid away from the water. It also meant that in contrast to the amphibians the reptiles could produce fewer eggs at any one time, because there was less risk of predation on the eggs.
Reptiles don't go through a larval food-seeking stage, but undergo direct development into a miniature adult form while in the egg, and fertilisation is internal.
The earliest date for development of the amniotic egg is about million years ago. However, reptiles didn't undergo any major adaptive radiation for another 20 million years. Current thinking is that these early amniotes were still spending time in the water and came ashore mainly to lay their eggs, rather than to feed. It wasn't until the evolution of herbivory that new reptile groups appeared, able to take advantage of the abundant plant life of the Carboniferous.
Early reptiles belonged to a group called the cotylosaurs. Hylonomus and Paleothyris were two members of this group. They were small, lizard-sized animals with amphibian-like skulls, shoulders, pelvis and limbs, and intermediate teeth and vertebrae. The rest of the skeleton was reptilian. Many of these new "reptilian" features are also seen in little, modern, amphibians which may also have direct-developing eggs laid on land e. New Zealand's leiopelmid frogs , so perhaps these features were simply associated with the small body size of the first reptiles.
A major transition in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian - million years ago , when the reptile group that included Dimetrodon gave rise to the "beast-faced" therapsids.
The other major branching, the "lizard-faced" sauropsids, gave rise to birds and modern reptiles. These mammal-like reptiles in turn gave rise to the cynodonts e. Thrinaxodon during the Triassic period. This lineage provides an excellent series of transitional fossils. The development of a key mammalian trait, the presence of only a single bone in the lower jaw compared to several in reptiles can be traced in the fossil history of this group.
It includes the excellent transitional fossils, Diarthrognathus and Morganucodon , whose lower jaws have both reptilian and mammalian articulations with the upper. Other novel features found in this lineage include the development of different kinds of teeth a feature known as heterodonty , the beginnings of a secondary palate, and enlargement of the dentary bone in the lower jaw. Legs are held directly underneath the body, an evolutionary advance that occurred independently in the ancestors of the dinosaurs.
The end of the Permian was marked by perhaps the greatest mass extinction ever to occur. Recent research has suggested that this event, like the better-known end-Cretaceous event, was caused by the impact of an asteroid.
During the subsequent Triassic period - million years ago , the survivors of that event radiated into the large number of now-vacant ecological niches. However, at the end of the Permian it was the dinosaurs, not the mammal-like reptiles, which took advantage of the newly available terrestrial niches to diversify into the dominant land vertebrates. In the sea, the ray-finned fish began the major adaptive radiation that would see them become the most species-rich of all vertebrate classes.
One major change, in the group of reptiles that gave rise to the dinosaurs, was in the animals' posture. This changed from the usual "sprawling" mode, where the limbs jut sideways, to an erect posture, with the limbs held directly under the body. This had major implications for locomotion, as it allowed much more energy-efficient movement. The dinosaurs , or "terrible lizards", fall into two major groups on the basis of their hip structure : the saurischians or "lizard-hipped" dinosaurs and the ornithischians misleadingly known as the "bird-hipped" dinosaurs.
Ornithischians include Triceratops , Iguanodon , Hadrosaurus , and Stegosaurus. Saurischians are further subdivided into theropods such as Coelophysis and Tyrannosaurus rex and sauropods e. Most scientists agree that birds evolved from theropod dinosaurs. Although the dinosaurs and their immediate ancestors dominated the world's terrestrial ecosystems during the Triassic, mammals continued to evolve during this time. Mammals are advanced synapsids. Synapsida is one of two great branches of the amniote family tree.
Amniotes are the group of animals that produce an amniotic egg i. The other major amniote group, the Diapsida, includes the birds and all living and extinct reptiles other than the turtles and tortoises. Turtles and tortoises belong in a third group of amniotes, the Anapsida. Members of these groups are classified on the basis of the number of openings in the temporal region of the skull. Synapsids are characterised by having a pair of extra openings in the skull behind the eyes. This opening gave the synapsids and similarly the diapsids, which have two pairs of openings stronger jaw muscles and better biting ability than earlier animals.
The jaw muscles of a synapsid are anchored to the edges of the skull opening. Pelycosaurs like Dimetrodon and Edaphosaurus were early synapsids; they were mammal-like reptiles. Later synapsids include the therapsids and the cynodonts , which lived during the Triassic. Cynodonts possessed many mammalian features, including the reduction or complete absence of lumbar ribs implying the presence of a diaphragm; well-developed canine teeth, the development of a bony secondary palate so that air and food had separate passages to the back of the throat; increased size of the dentary - the main bone in the lower jaw; and holes for nerves and blood vessels in the lower jaw, suggesting the presence of whiskers.
By million years ago the mammals had already become a diverse group of organisms. Some of them would have resembled today's monotremes e.
Until recently it was thought that placental mammals the group to which most living mammals belong had a much later evolutionary origin. However, recent fossil finds and DNA evidence suggest that the placental mammals are much older, perhaps evolving more than million years ago. Note that the marsupial and placental mammals provide some excellent examples of convergent evolution , where organisms that are not particularly closely related have evolved similar body forms in response to similar environmental pressures.
However, despite the fact that the mammals had what many people regard as "advanced" features, they were still only minor players on the world stage. As the world entered the Jurassic period - million years ago , the dominant animals on land, in the sea, and in the air, were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic, were the chief land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs.
Taking wing: Archaeopteryx and the origins of the birds. In an intriguing fossil was found in the Jurassic Solnhofen Limestone of southern Germany, a source of rare but exceptionally well-preserved fossils. Given the name Archeopteryx lithographica the fossil appeared to combine features of both birds and reptiles: a reptilian skeleton, accompanied by the clear impression of feathers.
This made the find highly significant as it had the potential to support the Darwinians in the debate that was raging following the publication of "On the origin of species". While it was originally described as simply a feathered reptile, Archaeopteryx has long been regarded as a transitional form between birds and reptiles, making it one of the most important fossils ever discovered. Until relatively recently it was also the earliest known bird.
Lately, scientists have realised that Archaeopteryx bears even more resemblance to the Maniraptora , a group of dinosaurs that includes the infamous velociraptors of "Jurassic Park", than to modern birds.
Thus the Archaeopteryx provides a strong phylogenetic link between the two groups. Fossil birds have been discovered in China that are even older than Archaeopteryx, and other discoveries of feathered dinosaurs support the theory that theropods evolved feathers for insulation and thermo-regulation before birds used them for flight.
This is an example of an exaptation. Closer examination of the early history of birds provides a good example of the concept that evolution is neither linear nor progressive. Also, a new species might evolve to compete with an existing species. Biologists are sure that once a species becomes extinct it never appears again. In the modern world, biologists can identify species by seeing whether the organisms can breed with one another.
Paleontologists have much more trouble with fossil species, because the organisms are no longer around to breed!
All that can be done is to match up shells or imprints that look almost identical and then assume that they represent a species. Paleontologists are sure that the fossil record is biased. That means that some kinds of organisms are much scarcer as fossils than they were when they were alive. Other kinds of organisms are much better represented by fossils. Animals with hard shells and skeletons are represented well in the fossil record.
On the other hand, soft-bodied animals are probably represented very poorly. It's likely that most soft-bodied species that ever existed are gone forever without a trace. Land animals are probably very poorly represented as well. For example, most animals that are now alive, or ever have lived, are insects, but the fossil record of insects is poor.
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