Deep Sea Killers-C. megalodon

Many consider the Great White Shark (Carcharodon carcharias) to be among the most incredible creatures to roam the oceans today. Growing up to a length of 6m (20ft), it is the infamous creature made famous by the success of the movie Jaws.

The Great White Shark as a species can be traced back to the early Pliocene (5 million years ago). It's fossilized teeth (Fig.1) can be found almost anywhere, worldwide, in marine sediments of the correct age. Teeth found in the Miocene (15 to 13.5 million years ago) are slightly rolled and have an eroded edge. Common synonyms for these teeth are Carcharodon rondeletti and Carcharodon sulcidens, but the teeth are identical to those of the living species not regarding intraspecific variation.

Fig. 1. Fossilized tooth of Carcharodon carcharias. Height: 5 cm. Early Pliocene, Sacaco/Peru © L. Andres

About 16 million years ago (during the Miocene), a distinct species appeared in the world's oceans. Carcharodon megalodon (or C. megalodon) was possibly the biggest shark species to have inhabited the oceans. It may have attained an astonishing maximum length of 15 m (50ft) and weighed as much as 50 tonnes (Fig.2)

Fig. 2. Comparing C. megalodon (13 m) and the Great White Shark (6.5 m) © L. Andres

Such estimates are obtained from teeth and certain skeletal components (as sharks have a skeleton made out of cartilage that does not fossilize easily; the teeth however are very durable). Traditional research holds C. megalodon as an ancestor of the white shark. Recent research suggests that it may have been a close relative. It's triangular teeth may have reached a maximum height of 17 cm (Fig.3). It may have hunted in the same stealthy manner as white sharks do, stalking beneath it's prey and rising upwards at great speeds to deliver a forceful, and often fatal, first bite.  It's prey probably included primitive whales and other large marine mammals.

Fig. 3. Fossilized tooth of Carcharodon megalodon. Height: 13 cm. Middle to Late  Miocene, Florida/USA © L. Andres

Around 1.5 million years ago (towards the end of the Pliocene), C. megalodon became extinct. The development of megatooth sharks can be traced back until the Cretaceous period. It is directly linked with the development of other animals. In the Cretaceous not only sharks, but also marine reptiles ruled the waters. This condition changed, however, after the extinction of the dinosaurs (65 million years ago) in favour of the sharks. Sharks now occupied the ecological niches for predators. Additionally, the basis for a more energy-rich nutrition was created by the rise of marine mammals (such as the cetoheriids; ancestors of baleen whales) in the Eocene (55 to 33 million years ago). During the Late Oligocene (30-25 mya) the climatic conditions were much more favourable. Temperatures were significantly higher than they are today, tropical and subtropical waters reached much further into the higher latitudes of the polar regions. During the following epoch, the Miocene (25-5 million years ago), modern baleen whales (Mysticeti) developed and spread more and more. There was an increase in size in whales and simultaneously in megatooth sharks. During the spreading out of the whales they presumably reached cooler polar waters that provided them with a richer food supply to which the whales adapted themselves. The whales were migrating between cool water feeding grounds and warm water breeding grounds. The climate became colder at the end of the Miocene and the beginning of the Pliocene (about 5 million years ago). The ice cover of the Antarctic polar region grew bigger and the mean sea level dropped. Living conditions and habitat for C. megalodon, who probably loved warm waters, obviously were restricted to such a degree that the species became extinct during the Pliocene. 

Reference:

C. megalodon-Megatooth Shark by Lutz Andres

Roesch, Ben S. 1998. A Critical Evaluation of the Supposed Contemporary Existence 
of Carcharodon megalodon. The Cryptozoology Review 3 (2): 14-24.

Evolution of scorpion venom

The predominant pharmacologically active components in scorpion venoms are small polypeptide molecules, usually basic in nature. Scorpion venoms and their component elements have been studied for over 35 years; but lately the focus in these studies has shifted from their pharmacological and electrophysiological properties to their molecular structures. This came about due to the realization that many classes of peptides seem to bind to characteristic spots on their targets, which for the most part are ion channels. The peptide toxins also show considerable identity in their arrangement of cysteine residues within the polypeptide chain. These cysteine residues are terribly important because disulphide bonds are one of the major contributors to conformational stability in small peptides.

The first pharmacological studies on scorpion toxins concentrated on their effects on mammalian model systems, so mice were the first ones to draw the short end of the hypodermic syringe, for a while. Peptides were consequently classified into the alpha- and beta-types, where each type bound to its own special site on voltage-gated sodium channels. Eventually, however, it was discovered that some peptides worked against ion channels in insects, and now Sarcophaga argyrostoma blowfly larvae were adopted as the victim model system of choice. All anti-insect toxins induced paralysis, but one class of toxins was found to induce a contraction paralysis, while the other induced a depressant paralysis.

For a while it seemed that all peptides could be neatly docketed in this way, but continuing studies unearthed peptide toxins that were primarily anti-mammal but showed some anti-insect activity too. Subsequently peptides were found that showed comparable anti-insect and anti-mammal activities, one of which – showing anti-insect as well as alpha-type and beta-type anti-mammal activities – was first reported in the paper (Loret et al., 1991) referenced below. This sparked off a line of thought on how the types of peptides found in scorpions in different parts of the world could be used to put forward theories about where scorpions first appeared and how they diverged into the large number of species we see today.

The reason this cross-reactivity of toxins against insect and mammal receptors was such a big deal was this: Before this was discovered, toxins were found to be easily classifiable not only on the basis of their primary peptide sequences and pharmacological activities, but also based on geography. Alpha-type anti-mammal toxins came only from Old World scorpions and beta-type anti-mammal toxins came only from New World scorpions. All anti-insect toxins had been purified from Old World scorpions only. Deviations from this geographical structure brought to light interesting ideas about venom evolution in scorpions.

For example, the toxin mentioned above which had a high effect on mammals but a low effect on insects was a beta-type toxin from Centruroides (a New World species) which was toxic to insects, but 50 times weaker than Old World anti-insect toxins (these studies happened in the 80s). This could indicate that anti-insect activity is just starting to evolve in New World venoms, but has already become established in Old World ones, indicating that the Old World venoms appeared first in their evolutionary history.

The paper referenced below is a report of a toxin from Androctonus australis Hector (pictured below), called AaH IT4. The toxin was purified by successive steps of chromatography, on gel filtration, DEAE-Sephadex and C8 HPLC columns. Toxicities were tested on S. argyrostoma larvae and male C57/BL6 mice, and the ED₅₀ values (for larvae) and LD₅₀ values (for mice) were recorded. Binding assays using ¹²⁵I-iodinated toxins on synaptosomal preparations from cockroaches and rats; radioimmunoassay assays to check for cross-reactions with rabbit antisera against known anti-insect, alpha-type and beta-type toxins; and circular dichroism analyses for structural data were carried out. In addition, sequence analysis and sequence alignment against various other scorpion venoms were carried out.

The experimental data showed that AaH IT4 competed with all three types of toxins for target-binding, and that it cross-reacted with the antibody against a beta-type toxin, indicating some structural similarity with the beta-toxin class. The dendrogram generated from the sequence alignment showed that AaH IT4 is more closely related to beta-type toxins than to either alpha-type or anti-insect toxins (which supports the result of the RIA experiment), although the divergence between AaH IT4 and the beta-type toxin lineage took place a long time ago. This an important point because beta-type toxins come from New World scorpions while AaH IT4 was purified from an Old World toxin, so any sequence similarity suggests that some relationship may exist between the two.

Another point to be made about AaH IT4 is that the sequence analysis shows an absence of the amino acid proline. Proline had previously been found in every purified and studied peptide scorpion toxin, and was suspected to play a role in the stability of their conformations, since proline is more conformationally restricted as compared to the other amino acids. One explanation for the binding of AaH IT4 to three different kinds of binding sites is that the absence of proline allows a certain amount of backbone flexibility which may allow the peptide to switch between conformational states that preferentially bind to each of the target sites.

The authors themselves hypothesize that since the evolution of insect-specific toxins is clearly advantageous to scorpions (for whom insects form a major part of the diet), it is possible that AaH IT4 represents the closest approximation available right now to some kind of ancestral toxin sequence/structure, which later diverged into anti-mammal and anti-insect varieties.

I think it’s worth mentioning, in addition, that the isolated AaH IT4 corresponds to 0.06% of the total protein in the venom, so it’s possible that the production of this particular venom component may have undergone some down-regulation over the years, as a result of the development of other, more specialised toxins.

Post-1991, of course, the issue of toxin classification has become even more complicated, and the venoms being studied now include those with anti-arthropod specificity, and the set of recognised targets have grown to include voltage- and ligand-gated potassium- as well as calcium-channels instead of just the initial emphasis on voltage-gated sodium-channels. Peptides with microbial activity have also been reported.

References:

Loret, E., Martin-Eauclaire, M., Mansuelle, P., Sampieri, F., Granier, C., & Rochat, H. (1991). An anti-insect toxin purified from the scorpion Androctonus australis Hector also acts on the .alpha.- and .beta.-sites of the mammalian sodium channel: sequence and circular dichroism study Biochemistry, 30 (3), 633-640 DOI: 10.1021/bi00217a007

Designed and Designoid Objects

Part 2 of the 5 part Royal Institution Christmas Lectures for Children, 1991. With sheer clarity and brilliance, Richard Dawkins puts forward the concept of objects that are designed and "look designed"

The Art of Evolving

Irreducible complexity is an argument that was first put forward by Michael Behe, a biochemist and proponent of intelligent design. Behe himself defines an irreducibly complex system as acollection of well-matched and interacting parts that on the whole, contribute to the functioning of the system, but the removal of any one part could cause the system to cease functioning. Most proponents of intelligent design and creationism like to argue in favor of irreducible complexity by using the eye as an example.  Even on the surface, the argument appears ridiculous and ignorant. Consider an individual suffering from a cataract. The crystalline lens of the person is affected to varying degrees (from slight to complete opacity). Equally, the vision too is affected to varying degrees. A slight cataract may prevent the individual from seeing as clearly as an individual with no cataract, but he or she can still see.

So what does evolution have to say? Well, having an eye is not a jackpot. It obviously isn't a case of black and white. There are shades of gray. Most evolutionary biologists argue that having something is better than having nothing. Having 10% of a complete eye is better than having no eye. or having 51% of an eye is better than having 49% of an eye. So how could the eye have evolved? Obviously, the earliest ancestors had no eyes (in the way that we know what an eye is), but they may have had a single layer of photo-sensitive cells (see Fig. 1a). Now the only thing such a layer of cells would have been able to do is to detect light and dark.  Not too much, but could've still assisted the animal when it had to detect if there were any predators around in the neighborhood. Euglena, for example, has flat patches of photoreceptors.

Now the next stage in evolution would have led to an indentation being formed in the photoreceptor layer, which gradually deepened to give rise to a cup-shaped structure (see Fig. 1b). This was better. Now this animal could not only distinguish light from dark, but also the direction from which the light was coming, since there is a shadow formed. Cup- shaped light sensitive spots are seen in Planaria.  As evolution proceeded, the cup got deeper and deeper and with even the slightest increase in depth, the animal was able to make out, with ever increasing accuracy, the direction of light. Later, the cup may have started to close at the other end, ultimately forming a roughly spherical structure with a hole at one end (see Fig. 1c). This structure would have been equivalent to a pinhole camera. One animal that does have a pinhole camera for an eye is Nautilus, a sea mollusk. Now the resolution of a pinhole camera depends on the size of the aperture. Smaller the aperture, sharper the image. But if the aperture is small, then the amount of light passing in would also be less and hence the image would be dim. Hence, in a pinhole camera, one is always compromised over the other.

In any case, Nautilus had a pinhole camera where the aperture was open. Sea water could constantly flow in and out. So as the process continued, the aperture would have been covered with a thin layer of transparent material (which wouldn't have contributed much at first except for protecting the eye), which may have been the earliest form of the cornea (see Fig. 1d). The cornea, as we know it today, has a refractive power of about 43 diopters and does contribute to focussing light (along with the lens). Gradually as evolution went on, the cornea from being just a sheet of transparent protective material may have gained some refractive properties, allowing sharper images to be formed (even when the size of the aperture was large). Although the cornea contributes the most to the focussing power of the eye, it's focus is fixed. What "tunes" the focus in response to objects at different distances is the curvature of the lens. And thus, was born the lens (see Fig 1e)!

We see how evolution beautifully explains the gradual formation of a complex structure and puts to rest two arguments, one being that of irreducible complexity and the other being that of the Watchmaker.