LIFE, MASS EXTINCTIONS, THE ANTHROPOCENE. 4th part (Plants and animals: the conquest of continents)

 

Post n. 43 English

1st part etichetta Zga, 2nd part etichetta Zha, 3rd part etichetta Zia

The Cambrian continents originated from the fracture of the supercontinent Rodinia 550 million years ago. When the animals originated, the continents were well separated and had given rise to a large surface area of shallow seas where animals could thrive. But the continents, formed at the south pole, did not remain stationary, they continued to migrate and about 300 million years later they reunited, but this time at the north pole giving rise to a new supercontinent: Pangaea.

It is clear that the movement of the continents must have brought about both locally and globally considerable climatic changes to which life had to adapt. We do not have much evidence of these changes, but through fossils and sediments, we can trace the dramatic history of living organisms over the last 500 million years and their evolution to the present day.

So, imagine going back in time and looking at the earth 500 million years ago. You would see oceans teeming with life: all kinds of single-celled organisms, soft-bodied animals and others with shells and armors of various shapes and sizes, and algae of enormous size. On the other hand, the continents were desolate lands, sunburned and made uninhabitable by ultraviolet rays. But between 500 and 450 million years ago begins a radical change. Perhaps due to a tidal wave or a sudden lowering of the seas, some algae were trapped in the reefs. Luckily for them in that period the increase of oxygen in the atmosphere and the consequent formation of ozone prevented the ultraviolet rays to reach the ground and destroy them. These algae adapted to the environment and over a few tens of millions of years, from mutation to mutation, through a progressive adaptation, they gave origin first to fungi and mosses. It is a fossil of a fungus, called Tortotubus, dated 442 million years ago, the oldest evidence of organisms that lived on land, ("At the dawn of life on land" Scientific American 2016 Will Dunham). According to a study by Cardiff University and published in the journal Geology around 430 million years ago, plants were only a few inches tall. Fungi and mosses extended further and further into the interior of the continents giving rise to the ancestors of ferns. These enormous green carpets, which reproduced through the production of spores, continuously sucked CO2 from the atmosphere in order to feed themselves, thus decreasing the greenhouse effect. As we wrote in the previous article, the alteration of the carbon cycle gave rise to periods of glaciation that led to the extinction of 60% of living organisms, the first mass extinction, the Ordovician extinction. But with the slow emission of CO2 by volcanism, the green army does not stop and with the appearance of plants with stems and leaves, within 200 million years, all continents were invaded by forests. The fossils of these huge forests gave origin to the period that geologists call Carboniferous, which goes from 360million years ago to 290 million years ago when the Permian begins.

  

 

While plants were advancing on land, marine animals, which had survived the Ordovician extinction, began to seize the opportunity of new food sources and began, through mutations, to adapt to the terrestrial environment. Insects and amphibious animals initially appeared living near marshes, capable of breathing both underwater and in the air. Vertebrate fish had to wait about 100 million years before they transformed their fins into legs. In fact, a fossil discovered in China that testifies to this evolutionary transition dates back to 395 million years ago.

Around 375 million years ago, living organisms underwent a decimation of 80%, the second mass extinction, the Devonian extinction. According to some researchers, the extinction occurred over 3 million years and we do not know the causes. According to other researchers, the extinction occurred over 50 million years. In the latter case, it would not be a true mass extinction but a decrease in biodiversity.

The planet maintained for a period of more than 100 million years a certain geological and climatic balance, but around the end of the Permian, 250 million years ago, occurred the most serious decimation of species in the history of the Earth, the third mass extinction, the Permian extinction. Douglas H. Hervin in "The mother of all extinctions" Le Scienze 1996 highlights how in a million years 90 percent of all species present in the oceans, two thirds of reptile and amphibian families and 30 percent of insects have become extinct. By analysing data from the chemism of the oceans of that era, fossils, and the Earth's crust dating from that time Erwin surmises that the Permian extinction was caused by a number of concomitant causes: the aggregation of continents that formed a single supercontinent, Pangaea, resulting in the disappearance of shallow water occupied by shallow sea communities; a lowering of sea levels, probably due to the widening of ocean basins, which destroyed nearshore habitats and destabilized the climate; an intense volcanic activity that began 255 million years ago and continued for several million years, which initially cooled the earth but led, in the long term, to a warming of the planet and the destruction of the ozone shield; finally hundreds of thousands of years later, sea level rose again, invading nearby habitats and destroying coastal communities.

Meanwhile, extraordinary biological changes were taking place among plants and among amphibians. As reported by biochemist and Nobel Prize winner Christian de Duve in "Vital Dust", to the environmental challenge brought about by the Permian disaster, life with appropriate adaptations turned disaster into success. Already during the Permian period, some plants to reproduce began to produce seeds, which can germinate anywhere, while the spores need humid environments. In the same period among amphibians, the female through mutations was the protagonist of the future.

  

Egg cell

Instead of releasing the egg cells and let them develop in the water, she enclosed them inside a container full of liquid: the egg appeared. And it was the egg that later allowed the animals to free themselves definitively from water and conquer the continents. The terrestrial reproduction had begun, the first reptile was born. These organisms, practically insignificant during the Permian, emerged victorious after the catastrophe and 250 million years ago began the conquest of the emerged lands. The main survivors of these reptiles are snakes and turtles, but the most famous and spectacular were the dinosaurs.

But even if from a biological point of view almost everything was ready, the appearance of the dinosaurs had to wait more than 50 million years because another catastrophe awaited the planet. Around 200 million years ago, before the advent of the dinosaurs, the fossil record tells us of a fourth mass extinction, the Triassic extinction that took away 50% of the living organisms. Even on this extinction, we still have no certainty.

According to a study of Ruthger University, as reported in "The dawn of the dinosaurs under the meteorites", Le Scienze 2002 that takes up an article by Kent and others, Ascent of Dinosaurs Linked to an Iridium anomaly at the Triassic-Jurassic Boundary. Science, May 17, 2002, it is possible to speculate that the cause of the Triassic extinction, which paved the way for the reign of the Dinosaurs, was the collision of the Earth with an asteroid. As explained by Dennis V. Kent, professor of Geology, in 70 sites in North America has been observed a concentration of Iridium, of sure space origin that exceeds the average values of our planet. Iridium, as we will see later, is a chronological marker.

Charles Choi, The Science 2012 "An asteroid impact also marked the beginning of the reign of the dinosaurs" (Scienticamerican.com 2012), reports that some researchers have identified an impact crater of about 40 km in diameter of an asteroid which fell at that time in Rochechouart in France, which would have produced a drastic climate change causing the extinction. But according to a group of researchers, in Science on line that takes up an article of Science express 2013, climate change would have been produced by apocalyptic volcanic eruptions that led to the shattering of the supercontinent Pangaea.

Lescienze.it

 

The extinction of the Triassic decimated living organisms, in particular disappeared many varieties of reptiles that freed ecological niches. It opened the way to the kingdom of dinosaurs.

The first dinosaur fossil dates back to 193 million years ago. Dinosaurs remained undisputed rulers of the continents for 130 million years until 65 million years ago when they suddenly disappeared and with them more than 50% of living organisms, the fifth mass extinction, the end of Cretaceous. They were animals that still fascinate people of all ages and many books and a huge number of scientific and popular articles have been written about them. In the last 50 years what has interested researchers and passionate common citizens the most has been the cause of their disappearance.

At the beginning of the 90's of the last century it was not yet clear the cause of the disappearance of Dinosaurs because in that period there were two catastrophic events: the fall of a meteorite and a volcanic eruption that lasted for hundreds of thousands of years. Since at that time the two hypotheses were theoretically acceptable, the controversy, initially, shifted to the timing.

As Vincent E. Courtillot writes in "A volcanic eruption" Le Scienze 1990 "Newly acquired data suggest that the mass extinction occurred in tens and perhaps hundreds of miles of years. Such a duration corresponds well to a period of violent volcanic eruptions that occurred in India, which gave rise to the formation of the Delcan traps, precisely at the time of the mass extinction".

Also in Le Scienze 1990 Walter Alvarez and Frank Asaro in "An extraterrestrial impact" supported, as deduced from the title, the second hypothesis. This hypothesis is based on the concentration of Iridium in some sediments.  Iridium is found in meteorites and asteroids, the same bodies that gave rise to the planets. During its formation, the earth was completely molten. When the earth cooled, Iridium sank into the core along with iron. The concentration of Iridium on the surface of our planet is therefore very low. In the seventies Alvarez examined a thin layer of sediments, dated 65 million years or KT limit (the limit between Cretaceous and Triassic), in Italy in Gubbio where it was found a concentration at least 300 times higher than the average found on the Earth's surface. And the authors write: "More than 100 scientists in 21 laboratories have found abnormally high levels of Iridium in the layers corresponding to the KT limit in about 95 locations around the world. [...] The iridium anomaly is well explained by the impact hypothesis [...] A 10 km asteroid moving at more than 10 km per second would cause a huge air pocket in the atmosphere.

 

Escape

On impact with the ground, the kinetic energy of the asteroid would be transformed into heat in a non-nuclear explosion 10 000 times more powerful than the one that could provoke the entire world arsenal of nuclear weapons. […]Others consider volcanism among the main clues. [...] the eruptions must have occurred in at least 500 000 years. Therefore a great part of the scholars has not considered the volcanism a serious "suspect" of the extermination that with every probability happened in 1000 years or less". Since a 10 km asteroid impact would have resulted in a crater about 150 km in diameter, the question shifted: and where is the crater?

At that time, the only known large crater was Manson in Iowa but it was 32 km in diameter, too small. Towards the end of the 90s of last century, after some clues surfaced in previous decades was discovered a crater 180 km in diameter at Chicxulub in the Yucatan Peninsula in Mexico. Residual deposits near the crater have been dated to the time of the disappearance of the dinosaurs. For most scientists the question seems therefore closed: it was the impact of an asteroid with the earth to cause the extinction of the dinosaurs that occurred, according to the latest research of palaeontologist Pincelli Hull, in the space of a few hundred years, but the volcanism responsible for the Delcan traps contributed to the mass extinction of the Cretaceous for at least another 200000 years. Finally, if it should be confirmed in the future the hypothesis of Dennis Kent that it was the impact with an asteroid to cause the extinction of the Triassic that opened the way to the dinosaurs, what to say, space makes and space unmakes and as Alvarez and Asaro write, natural selection is not the only driving factor of evolution, but it also takes a lot of luck. And we, who came out after the last impact, hope that history does not repeat itself. 

It is probable that dinosaurs were cold-blooded animals like today's reptiles, but at least one of their branches had to be warm-blooded, that is they were able to control body temperature between 36-38 ° C. In full reign of dinosaurs, new mutations, of which the female was still protagonist, created the seed of a new biological revolution. As reported again by Christian de Duve, the females of the branch of the warm-blooded dinosaurs, to protect the egg from predators, for the survival and propagation of the species, began to keep the egg inside their body until hatching and subsequently equip themselves with epidermal glands for the baby nutrition: mammary glands appear; mammals appear.

 

Lescienze.it

The mammals were practically irrelevant during the dominion of the dinosaurs, they reached at most the size of a rabbit, but at the fall of the dominators, they survived conquering the emerged lands.

The rest is history.                                                                     

 

                                                                         Giovanni Occhipinti

The next and final article on the subject will be published at the end of May