OUT OF THE WATER
‘You have to step backward, the better to jump forward.’
French proverb
French proverb
Around 500 million years ago (or ‘Once upon a time’?) while all of Earth’s complex organisms still lived in the water, a few algae clinging to rocks above the low-water mark took the huge evolutionary step that brought plant life onto land.
When exposed by low tides algae quickly dried out and in drought conditions they died. But now some were creating a waxy external layer that enabled them to stay wet on the inside until water covered them again. A few even adapted enough to live in moist conditions rather than in water, evolving into early mosses and liverworts.
By this stage a new kind of beings had emerged, plant-like organisms that do not photosynthesize and live on organic matter (the kingdom of fungi). A few merged with algae to form paler-coloured lichens, in which the algae supplied carbohydrates from photosynthesis, and the fungi supplied extra nourishing minerals.
Microbes played their part in this, for a number had come on land some time before and started breaking rock down into soil. Because it could store rainwater and minerals soil was essential for the first true land-plants to emerge.
Now a few creatures without backbones (invertebrates) began to crawl up and out from riverbeds and seabeds. They included big sea-worms, millipedes, mites and shelled and segmented insects, as well as predators like spiders, which hid under rocks or burrowed into the soil to wait for passing prey much as they had done in the water. But they all had to create ways of breathing, moving and breeding in air rather than in water, and so their various organs and the interaction among them evolved too.
At this stage a few small insects might have begun experimenting with living on the water-surface and even growing light wings.
Soon taller plants, like horsetails (with stiff leaves arranged in whorls) and ferns (with uncoiling fronds), were sending rootlets into the soil after nutrient-rich water. They could store it inside themselves now, pumping it up as sap into their strengthening, lengthening stems, built up from the stubs of fallen fronds or leaves. The taller they grew the more the breezes could help disperse their spores.
Slowly different plants spread from the fertile river deltas and coasts into the interior, and Earth’s barren land-surfaces began to go green. After transforming the water and atmosphere, life was now transforming the land.
But at this point Earth had its First Major Mass Extinction since complex life evolved, apparently associated with a huge ice age. Based on fossil records more than half of the animals on land and in the water seem to have completely disappeared, including many kinds of trilobites.
But what causes ice ages and other devastating climate changes?
The most popular theory today focuses on Earth’s tectonic plates. Their movement not only causes volcanic and earthquake activity, but also re-organizes Earth’s land-masses and oceans. All this leads to recurrent, often quite sudden, climate changes as a tipping-point is reached. And depending on their scale, local or widespread extinctions follow.
However an overwhelming climate change can come about in many ways. Some are triggered by inorganic events such as variations in Earth’s orbit or tilt or asteroid impacts, and some by activities of organisms, such as the release of poisonous oxygen by blue-green bacteria in search of food (see the post of Our First Age, Scene 3).
Earth has had five Major Mass Extinctions, and many minor ones. After each one life has eventually reassembled in more complex and diverse ecosystems than before. In this process more general adaptations such as sexual reproduction are usually carried over, but specific ones such as various kinds of visual perception apparently start all over again.
The transition to a new stability takes millions of years and scientists today have a fair idea how this happens.
As you might expect by now, microbes find it quite easy to adjust and keep on multiplying. Many recycle the bodies of the victims into new life, while others provide food for the survivors. Meanwhile the survivors keep reproducing, with gradual small changes to suit their new environment.
Over time lots of new plants and animals emerge, competing for available niches. If a large plant-eating family goes extinct, another large plant-eating family emerges, and so on.
Eventually the few winners in the transition game form new webs of life in each shared territory – and they keep on consolidating the new ecosystem together, until it is dissolved and replaced in its turn.
So these Major Mass Extinctions are another example of our Universe's system of dynamic balance punctuated by sudden transformational change.
It is as if the web of life on Earth (rather like a star) needs a sudden collapse so that new clumpings can occur. Altogether, as a result of both major and minor mass extinctions, scientists estimate that 99% of all species evolving on Earth are extinct today.
But there is an important difference between a major mass extinction and a star coming to the end of its fusion process.
The extinction event is not predictable, and the survival of any species has little to do with is fitness for life in a local ecosystem. External conditions have intervened, and the only pattern seems to be that species with a wide range of food options and/or a short life-cycle are more likely to survive.
On the other hand Earth’s evolution cannot go on forever. The Sun will exhaust its fuel in 4 or 5 billion years. And long before that, the searing expanding Sun will make it too hot for Earth to support life as we know it. Gaia will begin to die.
As the water evaporates more and more rapidly, evolution will turn back on itself as diversity and complexity decrease. First to go will be animals, then plants and other complex life, then all but the hardiest microbes.
At some time also the Moon will escape from its constantly widening orbit around Earth.
Then as the Sun rapidly expands before its final collapse, the probability is that our planet’s solids will melt or disintegrate, and solids and gases will be dispersed.
We presume that Earth’s energy will be transformed in some way, because scientists tell us that energy cannot be destroyed. And its rocks, dust and gases may become rocks, dust or gases somewhere else.
But perhaps some of the debris will harbour microbes in a dormant form, and they may colonize a moon circling Jupiter or Saturn. The hotter Sun will have thawed the ice there today and enable some to photosynthesize. Then the evolutionary impetus may lead to a new range of co-evolving beings there, surviving and thriving until the sunlight dims.
Earth’s microbes even have a very small chance of reaching a far distant solar system. If they manage to evolve in a very different environment, we can assume that it will be as a very different range of complex beings.
However there is no need for us to worry about all of this. Earth is still in the prime of its life.
Scientists expect that the cycling of major mass extinctions followed by increased biodiversity will continue on our planet for another billion years. That is the same time-frame in which humans evolved from the first multi-celled organisms – soft-bodied, slowly drifting and not yet plants or animals (see the post of Our First Age, Scene 4).
We are already exploring our solar neighbourhood. Surely, at some stage in the future, we will develop the technology needed to find a home in a younger solar system. Or if not us, then whatever species emerges in our place. And the long evolution that began on Earth will have a chance to continue there.
But back in our story, as microbes, plants and animals recovered from the First Major Mass Extinction, they tried many different ways to prosper in a range of new environments – on land, and in the air. By around 400 million years ago, among the early fish diversifying in the oceans there were now many with teeth, including at least one 6-metre long monster of the deep, Dunkleostus terrelli (long extinct) and also the coelacanth (still clinging to life today).
And several smaller species of toothed fish were colonizing tidal areas and shallow rivers. In the process the gill slits of some evolved into a set of lungs, and the bony fins of others into four stubby legs.
(One such creature, Acanthostega gunnari, lived in the area that would become Queensland. It had both gills and lungs and also four legs with digits, which it used in water and on land. Today’s lungfish is a descendant of a species like this.)
Soon, in response to lower water levels, lack of food or more efficient predators, a few walking fish followed invertebrates onto higher ground and evolved into amphibians, living both in water and on land. And the focus of our story narrows to follow them.
It is not too hard to imagine the enormity of the change involved in being one of those animals that emerged from the water, breathing and moving in air for the first time. Because of course we have had a related experience – as a baby emerging from warm security in the amniotic fluid of our mother’s womb.
But in order to manage the change these pioneers had to undergo what scientists call an incomplete metamorphosis, where they developed from eggs laid and fertilized in the water through fishlike young into air-breathing adults walking on dry land. As they spent more time on land they gradually evolved.
In due time they had a neck, a strengthened backbone, more agile limbs, a water-retentive skin, and a stronger, more efficient heart. And their brain developed accordingly: managing their breathing, helping them stand up to gravity, and improving their vision and hearing.
It also developed areas to handle dominance and submission postures, which they could use in encounters with a competing amphibian or a predator to avoid conflict.
But they didn’t yet have the ability to regulate their body temperature, and so they were active in the day, and slept at night.
When exposed by low tides algae quickly dried out and in drought conditions they died. But now some were creating a waxy external layer that enabled them to stay wet on the inside until water covered them again. A few even adapted enough to live in moist conditions rather than in water, evolving into early mosses and liverworts.
By this stage a new kind of beings had emerged, plant-like organisms that do not photosynthesize and live on organic matter (the kingdom of fungi). A few merged with algae to form paler-coloured lichens, in which the algae supplied carbohydrates from photosynthesis, and the fungi supplied extra nourishing minerals.
Microbes played their part in this, for a number had come on land some time before and started breaking rock down into soil. Because it could store rainwater and minerals soil was essential for the first true land-plants to emerge.
Now a few creatures without backbones (invertebrates) began to crawl up and out from riverbeds and seabeds. They included big sea-worms, millipedes, mites and shelled and segmented insects, as well as predators like spiders, which hid under rocks or burrowed into the soil to wait for passing prey much as they had done in the water. But they all had to create ways of breathing, moving and breeding in air rather than in water, and so their various organs and the interaction among them evolved too.
At this stage a few small insects might have begun experimenting with living on the water-surface and even growing light wings.
Soon taller plants, like horsetails (with stiff leaves arranged in whorls) and ferns (with uncoiling fronds), were sending rootlets into the soil after nutrient-rich water. They could store it inside themselves now, pumping it up as sap into their strengthening, lengthening stems, built up from the stubs of fallen fronds or leaves. The taller they grew the more the breezes could help disperse their spores.
Slowly different plants spread from the fertile river deltas and coasts into the interior, and Earth’s barren land-surfaces began to go green. After transforming the water and atmosphere, life was now transforming the land.
But at this point Earth had its First Major Mass Extinction since complex life evolved, apparently associated with a huge ice age. Based on fossil records more than half of the animals on land and in the water seem to have completely disappeared, including many kinds of trilobites.
But what causes ice ages and other devastating climate changes?
The most popular theory today focuses on Earth’s tectonic plates. Their movement not only causes volcanic and earthquake activity, but also re-organizes Earth’s land-masses and oceans. All this leads to recurrent, often quite sudden, climate changes as a tipping-point is reached. And depending on their scale, local or widespread extinctions follow.
However an overwhelming climate change can come about in many ways. Some are triggered by inorganic events such as variations in Earth’s orbit or tilt or asteroid impacts, and some by activities of organisms, such as the release of poisonous oxygen by blue-green bacteria in search of food (see the post of Our First Age, Scene 3).
Earth has had five Major Mass Extinctions, and many minor ones. After each one life has eventually reassembled in more complex and diverse ecosystems than before. In this process more general adaptations such as sexual reproduction are usually carried over, but specific ones such as various kinds of visual perception apparently start all over again.
The transition to a new stability takes millions of years and scientists today have a fair idea how this happens.
As you might expect by now, microbes find it quite easy to adjust and keep on multiplying. Many recycle the bodies of the victims into new life, while others provide food for the survivors. Meanwhile the survivors keep reproducing, with gradual small changes to suit their new environment.
Over time lots of new plants and animals emerge, competing for available niches. If a large plant-eating family goes extinct, another large plant-eating family emerges, and so on.
Eventually the few winners in the transition game form new webs of life in each shared territory – and they keep on consolidating the new ecosystem together, until it is dissolved and replaced in its turn.
So these Major Mass Extinctions are another example of our Universe's system of dynamic balance punctuated by sudden transformational change.
It is as if the web of life on Earth (rather like a star) needs a sudden collapse so that new clumpings can occur. Altogether, as a result of both major and minor mass extinctions, scientists estimate that 99% of all species evolving on Earth are extinct today.
But there is an important difference between a major mass extinction and a star coming to the end of its fusion process.
The extinction event is not predictable, and the survival of any species has little to do with is fitness for life in a local ecosystem. External conditions have intervened, and the only pattern seems to be that species with a wide range of food options and/or a short life-cycle are more likely to survive.
On the other hand Earth’s evolution cannot go on forever. The Sun will exhaust its fuel in 4 or 5 billion years. And long before that, the searing expanding Sun will make it too hot for Earth to support life as we know it. Gaia will begin to die.
As the water evaporates more and more rapidly, evolution will turn back on itself as diversity and complexity decrease. First to go will be animals, then plants and other complex life, then all but the hardiest microbes.
At some time also the Moon will escape from its constantly widening orbit around Earth.
Then as the Sun rapidly expands before its final collapse, the probability is that our planet’s solids will melt or disintegrate, and solids and gases will be dispersed.
We presume that Earth’s energy will be transformed in some way, because scientists tell us that energy cannot be destroyed. And its rocks, dust and gases may become rocks, dust or gases somewhere else.
But perhaps some of the debris will harbour microbes in a dormant form, and they may colonize a moon circling Jupiter or Saturn. The hotter Sun will have thawed the ice there today and enable some to photosynthesize. Then the evolutionary impetus may lead to a new range of co-evolving beings there, surviving and thriving until the sunlight dims.
Earth’s microbes even have a very small chance of reaching a far distant solar system. If they manage to evolve in a very different environment, we can assume that it will be as a very different range of complex beings.
However there is no need for us to worry about all of this. Earth is still in the prime of its life.
Scientists expect that the cycling of major mass extinctions followed by increased biodiversity will continue on our planet for another billion years. That is the same time-frame in which humans evolved from the first multi-celled organisms – soft-bodied, slowly drifting and not yet plants or animals (see the post of Our First Age, Scene 4).
We are already exploring our solar neighbourhood. Surely, at some stage in the future, we will develop the technology needed to find a home in a younger solar system. Or if not us, then whatever species emerges in our place. And the long evolution that began on Earth will have a chance to continue there.
But back in our story, as microbes, plants and animals recovered from the First Major Mass Extinction, they tried many different ways to prosper in a range of new environments – on land, and in the air. By around 400 million years ago, among the early fish diversifying in the oceans there were now many with teeth, including at least one 6-metre long monster of the deep, Dunkleostus terrelli (long extinct) and also the coelacanth (still clinging to life today).
And several smaller species of toothed fish were colonizing tidal areas and shallow rivers. In the process the gill slits of some evolved into a set of lungs, and the bony fins of others into four stubby legs.
(One such creature, Acanthostega gunnari, lived in the area that would become Queensland. It had both gills and lungs and also four legs with digits, which it used in water and on land. Today’s lungfish is a descendant of a species like this.)
Soon, in response to lower water levels, lack of food or more efficient predators, a few walking fish followed invertebrates onto higher ground and evolved into amphibians, living both in water and on land. And the focus of our story narrows to follow them.
It is not too hard to imagine the enormity of the change involved in being one of those animals that emerged from the water, breathing and moving in air for the first time. Because of course we have had a related experience – as a baby emerging from warm security in the amniotic fluid of our mother’s womb.
But in order to manage the change these pioneers had to undergo what scientists call an incomplete metamorphosis, where they developed from eggs laid and fertilized in the water through fishlike young into air-breathing adults walking on dry land. As they spent more time on land they gradually evolved.
In due time they had a neck, a strengthened backbone, more agile limbs, a water-retentive skin, and a stronger, more efficient heart. And their brain developed accordingly: managing their breathing, helping them stand up to gravity, and improving their vision and hearing.
It also developed areas to handle dominance and submission postures, which they could use in encounters with a competing amphibian or a predator to avoid conflict.
But they didn’t yet have the ability to regulate their body temperature, and so they were active in the day, and slept at night.
* * * * * *
Diagram of the Five Major Mass Extinctions on Earth
Diagram of the Five Major Mass Extinctions on Earthshowing how all except the Third (the most devastating one yet)
were followed by increasing diversity
(another Google Image)
were followed by increasing diversity
(another Google Image)
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