First Animals

Update #6 to Human Origins: How diet, climate and landscape shaped us

John S. Compton (www.johnscompton.com)

The sudden, but relatively late appearance of animals in the rock record has both fascinated and puzzled scientists for years. Simple bacteria emerged as early as 4000 million years ago, soon after Earth had become habitable (refer to my previous blog: First Life), but complex animals appeared only around 600 million years ago and burst onto the scene from 541 million years ago as part of the ‘Cambrian Explosion.’ The delay in the evolution of animals is usually attributed to low levels of oxygen gas in Earth’s atmosphere, which had to first rise above a threshold value before animals could proliferate. This idea finds support from rock record, which indicates that the arrival of animals coincides broadly with when oxygen levels increase sharply. But why did oxygen levels remain so low for so long, and what was it that led to our oxygen-rich atmosphere and the rapid evolution of diverse animal life?

rise of oxygen

Earth initially had no oxygen gas in its atmosphere, and oxygen remained at low levels (<0.1%) during the ‘boring billion’ years before it rapidly increased between 600 and 500 million years ago coinciding with the appearance of the earliest animals (adapted from Sperling and others, 2015).

In my book I state that we animals may owe thanks to the algae for our existence. This concept finds support in a recent paper by Brocks and others (2017), in which they show that the transition to an animal world corresponds to when algae suddenly became the dominant primary producers. Primary producers form the base of the food chain by using sunlight to grow by photosynthesis.  Oxygen gas is a by-product of photosynthesis, a biological process that is ultimately responsible for our oxygen-rich atmosphere. They propose that greater availability of the nutrient phosphorus in the world’s oceans allowed algae to dominate for the first time over bacteria as primary producers. Algae are significantly larger and more complex than bacteria, and as a result algae are more likely to end up buried in sediment before they react with oxygen and are converted back into carbon dioxide. Hence, the ‘rise of algae’ resulted in a rise in oxygen levels through enhanced burial of organic matter. The scenario they propose is a good example of how interactions between the living and non-living worlds may have promoted ecological transformations that ultimately led to the emergence of the vast animal kingdom to which we belong. What is the evidence for the rise of algae?

There are a number of ways in which life can leave evidence of its existence in the rock record. Body fossils are the preserved remains of the actual animal and they most commonly consist of hard parts, such as teeth, bone or shell. In rare cases the rapid draping of fine sediment soon after death can preserve soft tissue structures (Lagerstätte). Trace fossils, such as footprints, burrows or tooth marks can also provide evidence, although linking them to a specific animal can be difficult. Molecular fossils are distinct chemical compounds made by organisms referred to as biomarkers. Biomarker molecules, if stable can indicate the existence of an organism or group of similar organisms even in the absence of any other fossil evidence. For example, compounds derived from steroids or sterols (called steranes) have been used to indicate when the first sponges appeared (Love and others, 2009).  Sterane biomarkers specific to algae were used by Brocks and others (2017) to document when algae became abundant in the rock record. Importantly, their methods were designed to minimise contamination by petroleum products.

What the biomarker record of Brocks and others (2017) suggests is that, although algae had first appeared by around 1800 million years ago, algae only became dominant much later by 659-645 million years ago. Algae have far more complex, eukaryotic cells compared to bacteria (prokaryotic cells) and by 1600 to 1200 million years ago algae were the earliest multicellular organisms, having specialised cells for attachment, vertical elements and reproduction. These features represent major evolutionary innovations, and yet for all their innovativeness the algae do not appear to have displaced the bacteria as primary producers. Why not? In modern oceans bacteria tend to dominate in nutrient-poor waters, whereas algae take over once nutrients become more abundant. So, one possibility is that the early ocean had few nutrients, such as phosphorus, and that algae struggled to compete with bacteria until the nutrient content of the oceans increased. If this scenario is correct, then what could have increased the nutrient content of the oceans allowing algae to out compete bacteria?

red algae fossils

Fossil red algae 1200 million years ago grew in vertical filaments attached to a firm substrate and had reproductive structures (two images on the right ) (images courtesy of Nicholas Butterfield).

Snowball Earth is an appropriate name for a most unusual period of Earth history, the Cryogenian, when our planet experienced extreme climate cycles of cold, near complete icing over to hot climates when all the ice rapidly melted. Significant variations in ice and climate are known from the past, but none was nearly as intense as the hot and cold cycles of the Cryogenian. Prior to Snowball Earth oxygen levels were steady and low (<0.1% compared to 21% today) from roughly 1800 to 800 million years ago, a period referred to as the ‘boring billion’ when Earth was locked into a low-oxygen atmosphere. A low-oxygen atmosphere may partly explain why the nutrient content of the ocean was also kept low for so long. Under low oxygen conditions, the ocean has more iron and iron can keep surface, sunlit surface waters where photosynthesis occurs low in phosphorus by removing it through adsorption to iron oxides. Whatever the reasons for its long stability, the ‘boring billion’ finally came to an end with the onset of Snowball Earth. The large ice sheets ground large amounts of rock into fine powder that then underwent intense chemical weathering in the ice-free hot climates that ensued. If this weathering released large amounts of phosphorus to the ocean, then it may have spurred on the algae who, having waited patiently in the wings for so long could now take off and displace bacteria as the dominant primary producers.

snowball earth

Snowball Earth is when our planet cycled between cold, ice-covered intervals (centre) and hot, ice-free climates (far left and right). The position of the continents was different then compared to today, with most positioned near the equator where intense weathering may have contributed to initiating Snowball Earth. Input of carbon dioxide, a greenhouse gas, by volcanoes (dark streaks in centre image) eventually warmed Earth and the ice melted.

The dominance of algae as primary producers was a major event and one that has endured ever since, most probably because it established a powerful feedback loop that rapidly led to an oxygen-rich atmosphere. Snowball Earth cycles released more nutrients, more nutrients fuelled more growth of large multicellular algae, some parts of which were more resistant to degradation than others and were more easily buried. Burial of more algal organic matter in turn allowed more oxygen to remain in the atmosphere. Higher oxygen levels resulted in an iron-poor, but nutrient-rich ocean, which promoted the growth and burial of algae and a continued rise in oxygen, rapidly exceeding the threshold level at which animals could thrive. Algae also promoted the evolution of animals by providing a large source of food. Single-celled animals feed on tiny bacteria but large, multicellular animals could fed on algae as well as on other animals consuming the algae. The result was a major global ecological shift to far more complex and intricate food chains cascading up from the algal primary producers.

The timing fits nicely with the rock record. The increase in algal biomarkers occurs between the major icing over episodes of the Cryogenian and coincides with a large, positive carbon isotope shift indicating more efficient organic matter burial just prior to the emergence of the earliest animals. The earliest animals include sponges, jelly fish and odd, pillow-like animals of the Ediacaran fauna that evolved around 600 million years ago and that by 541 million years ago were joined by diverse bilaterian animals, the dominant animals on Earth ever since. Thus, it took a major disruptor in the form of Snowball Earth to knock the biosphere into a new level of complexity driven by a greater flux of nutrients, more organic matter burial, an oxygen-rich atmosphere and more diverse, multicellular animals. If this scenario is correct then we humans, along with all the other animals living today owe thanks to the algae, who waited patiently for the conditions to arrive that allowed them to proliferate and in so doing ushered in the animal world.

early life synopsis

Synopsis of the major events in the early evolution of life on Earth up to the emergence of complex animals (time is shown on the far right in billions of years ago).

 

Further reading

Brocks, J.J., and others, 2017. The rise of algae in Cryogenian oceans and the emergence of animals. Nature 548, 578-581 (doi:10.1038/nature23457).

Butterfield, N.J., 2002. Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology 26, 386-404.

Knoll, A.H., 2017. Food for early animal evolution. Nature 548, 528-530.

Love, G.D., and others, 2009. Fossil steroids record the appearance of Demospongiae during the Cryogenian period. Nature 457, 718-721 (doi:10.1038/nature07673).

Sperling, E.A. and others, 2015. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature 523, 451-454-721 (doi:10.1038/nature14589).

© John S. Compton (www.johnscompton.com)

 

 

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First Life (Inevitable Life?)

Update #5 to Human Origins: How diet, climate and landscape shaped us

John S. Compton (www.johnscompton.com)

Several recent papers present evidence that life was established early on, soon after Earth’s fiery and cataclysmic assembly had ended and its surface had cooled down into a watery world capable of sustaining life. Such an early start implies that life, rather than being unique to Earth may be, if not inevitable, then at least a highly likely outcome here as well as on other Earth-like planets. Give life half a chance and it will rise up and thrive. A result which perhaps shouldn’t come as too big a surprise considering the tenacity with which life has persisted and diversified over the many billions of years since it first emerged. One way to ascertain if life on Earth was a highly likely or inevitable outcome is to find it elsewhere. The recent discovery of thousands of distant planets (exoplanets) orbiting within the watery, and hence potentially habitable zone of their stars is a promising sign that we may be closing in on evidence that life does exists out there in the great beyond. Confirming life on just one exoplanet would imply that our universe is home to a multitude of living worlds peppered across its enormous expanse. This is a mind-boggling concept even if our chances of ever directly interfacing with life elsewhere is remote given the distances involved. As our search for life on distant planets continues, we can also look long and hard for evidence of the earliest life here on Earth. Establishing when life first appeared on Earth can provide us with valuable insights into how life might have come about, but finding convincing evidence of life in the ancient rock record is a difficult task.

The first problem is that most life never leaves even a hint of its existence in the rock record. Life is fragile and readily decomposes such that most living things upon death rapidly vanish, recycled back into the world of non-living elements. Hence, fossil remains of life are rarely preserved in rocks of any age, never mind long ago when the abundance and types of life forms were far more limited than today. The other problem is that older rocks become increasingly difficult to find the further back in time you go. This is because Earth is dynamic, with the movement of its crustal plates (plate tectonics) constantly recycling older rocks into younger rocks. The evidence of life can only be found in the rock record and, therefore, the oldest rocks define the limit of how far back we can potentially detect life. And even for those oldest rocks that we are lucky enough to find and that once harboured fossils, most have since been subjected to intense pressures and temperatures over the dynamic history of their long existence. Such abuse is likely to obscure, if not destroy, whatever original fossil evidence the now transformed rocks may have once contained. Despite these problems, what does the rock record reveal about the earliest life on Earth?

The oldest rocks yet found are around 4 billion years old and occur in Canada and Greenland. We know from the age and composition of meteorites that our solar system formed initially 4.56 billion years ago. It was at this time that Earth’s assembly began by the collision of the many small rocky and metallic bits circling the Sun in our planet’s orbit. Initially gravity pulled in surrounding dust and rock to form small planets (planetissmals) and then these collided gradually to form larger planets. The final, major planetissmal collision that went into the making of Earth occurred 4.44 billion years ago. Our Moon formed from the debris of this collision and the force of the collision transformed Earth into a magma ball, too hot to sustain life. Life could have conceivably been established once the magma ball had cooled and crusted over, and the water vapour had condensed to form the oceans. Conditions on Earth however remained challenging to life as it had to contend with periods of intense bombardment by the sweeping up of the remaining rocky bits. This time of heavy meteorite bombardment is known as the Hadean, named after Hades, god of the underworld. The intense bombardment and the Hadean ended 4 billion years ago to give way to the Archean. To whatever extent life may have started during the Hadean, it was only by the start of the Archean that Earth presented a relatively stable setting for life to persist.

hellish hadean

Archean Earth
The Hadean (top) was a hellish period of intense meteorite bombardment and vaporised oceans, conditions that were generally not favourable to life, while the Archean world (bottom) included oceans of water and conditions suitable for the earliest microbial life forms.

 

The oldest, direct fossil evidence of life is in the form of stromatolites. Stromatolites are mound-shaped structures that form from the activities of microbial mat communities. Stromatolites are common throughout much of the early rock record and some still form today in places like Shark Bay, Western Australia. The minute microorganisms themselves are rarely preserved, but the layered stromatolite structures they build as a result of their activities often are preserved. The distinctive features of stromatolites rule out alternative, non-biological origins for the layered structures, such as the deposition of mineral crusts. The oldest stromatolites were recently discovered in rocks from Greenland 3.7 billion years old (Nutman and others, 2016), significantly older than the previously oldest stromatolites known from Australia dated to 3.48 billion years.

1-Nutman stromatolites
Small conical-shaped mounds (dashed lines) in rocks from Greenland 3.7 billion years old are interpreted to be stromatolites formed by microbial communities (photo by Allen Nutman adapted from Allwood, 2017).

Going back beyond 3.7 billion years, the evidence of life relies on the chemical signature of what is interpreted to be the remains of once living organisms as well as possible fossils. Life on Earth is carbon based and hence most organisms are made up of a significant amount of carbon. For life that grows by taking up carbon directly as CO2, the lighter carbon isotope (carbon 12) is taken up preferentially to the heavier isotope (carbon 13) (the heaviest isotope (carbon 14) is radioactive and has a short life span). The preferential uptake of the light isotope gives life a distinct, negative carbon isotopic composition. When organisms die and their organic matter decomposes, most of the other elements besides carbon, such as nitrogen, phosphorus, oxygen and hydrogen are lost and eventually all that remains is carbon. When heated up under pressure the remaining organic carbon can form the mineral graphite (a soft, grey mineral that is familiar to us as pencil lead). At extreme pressures graphite can transform to diamond. Some have found organic matter in rocks from Greenland greater than 3.7 billion years old that still contains some oxygen, hydrogen, nitrogen and phosphorus because it was preserved as inclusions within the metamorphic mineral garnet (Hassenkam and others, 2017). But in most cases all that remains is pure carbon in the form of graphite. Graphite from rocks in Labrador Canada greater than 3.95 billion years old have carbon isotope values that suggest the carbon was originally part of living organisms. Some of the graphite takes on globular shapes common to some microorganisms (Tashiro and others, 2017). Structures in rocks at least 3.77 billion year old from Quebec Canada interpreted to represent fossil microorganisms suggest that mid-ocean ridge submarine hydrothermal vents or ‘black smokers’ are a potential setting in which early life evolved on Earth (Dodd and others, 2017). Unfortunately, the structures interpreted to be fossil bacteria and the carbon isotope values of graphite interpreted to indicate life processes can both be produced by reactions that do not involve living organisms. Therefore, the indications of life earlier than the 3.7-billion-year-old stromatolites are compelling, but remain ambiguous.

3.95 Ga graphite spheres

black smoker
Cluster of globular graphite in quartz chert from Canada is suggestive of fossilised microbial organisms (top, from Tashiro and others, 2017). A modern submarine volcanic vent or ‘black smoker’ is a possible setting where life first emerged on Earth (image from NOAA).

 

The oldest known rocks are around 4 billion years old, so how can we say anything about earlier periods of Earth history? Some younger rocks contain mineral grains derived from the erosion of older rocks. Zircon is a highly durable mineral that often ends up being eroded out of older rocks but is not destroyed by weathering and ends up being recycled into younger rocks. In fact, the oldest known mineral is zircon dated to as old as 4.38 billion years in rocks from Jack Hills, Western Australia. A small number of these zircon grains have minute inclusions of graphite. The graphite in a zircon grain dated to 4.1 billion years has a carbon isotope signature consistent with a biological origin, but again not unambiguously (Bell and others, 2015). Therefore, the recent evidence pushes back the earliest life on Earth from around 1000 million years to at least 700 million and possibly 300 million years following the final major collisional event 4.44 billion years ago. Assuming it took on the order of 100 million years for Earth to become habitable after the final major collisional event, reduces the time span of life’s emergence to between 600 and 200 million years.

timeline early Earth
Timeline of early Earth history from its formation as a molten, magma ball, through the hellish Hadean and into the Archean. Oldest direct fossil evidence of life is from stromatolites in rocks 3.7 billion year old and indirect chemical evidence of life comes from rocks around 4 billion year old and from graphite inclusions in zircon minerals 4.1 billion years old.

What is the significance of the suggested narrowing of the time it took life to emerge on Earth? The more rapidly life was established, the less difficult or improbable it would appear to be. The origin of life remains a major unknown. We know that early Earth likely had an enormous diversity of chemical compounds to work with including amino acids, which are the building blocks of all life today. But whether life was inevitable or even highly likely to come about is open to debate, because it remains unclear how these compounds became organised into self-replicating, evolving organisms. It could be that the emergence of life is simply too slow a process for us to replicate in the lab. Even the simplest of life forms, bacteria, involve an incredibly complex array of biochemical reactions and processes that would have taken time to evolve from the diverse pool of chemical compounds available. Although less than previously thought, several hundred million years is still a long span of time over which life could have gradually emerged.

It seems unlikely that the rock record will allow us to refine the timing much more or to push it back much further. Thus, the implied ease at which life can emerge on a place like Earth will perhaps have to wait for the discovery of life on another planet. Just what form life will take on other planets is unknown, but if at all like Earth, life is most likely to be dominated by diverse yet small, simple microbial organisms. Perhaps quick to get started, life on Earth appears to have remained fairly simple for a long period of time. It took around a billion years before the more complex eukaryotic cells evolved and another two billion years or so after that before animals evolved. The delay in the evolution of animals has been attributed to the need of an oxygen-rich atmosphere.  For the latest ideas on what may have controlled oxygen levels in the atmosphere and how the level became high enough for animals, check out my next blog update #6: First Animals.

Further reading

Allwood, A.C., 2016. Evidence of life in Earth’s oldest rocks. Nature 537, 500-501 (doi:10.1038/nature19429).

Bell, E.A., and others, 2015. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. PNAS 112, 14518–14521 (www.pnas.org/cgi/doi/10.1073/pnas.1517557112).

Dodd, M.S., and others 2017. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature 543, 60-64 (doi:10.1038/nature21377).

Hassenkam, T., and others, 2017. Elements of Eoarchean life trapped in mineral inclusions. Nature 548, 78-81 (doi:10.1038/nature23261).

Nutman, A.P., and others, 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature 537, 535-538 (doi:10.1038/nature19355).

Tashiro, T., and others, 2017. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada. Nature 549, 516-518 (doi:10.1038/nature24019).

© John S. Compton (www.johnscompton.com)

 

First and forever a microbial world

They are so small, we rarely give them a thought and yet they are out there in numbers too enormous for us to comprehend. In fact, they are estimated to constitute no less than half of the total mass of all life on Earth. So, when you next look out over the landscape covered by plants and teeming with insects, birds and animals, think of the roughly equivalent mass of life that you don’t see. This invisible other half of life is the microbial world, made up of a great variety of microorganisms, so called because they typically measure only 0.5 to 5 micrometres across (that is about one-thousandth to one-hundredth the width of the 0.5 millimeter sized dot atop the letter i).  Although they are nearly everywhere, to see them requires a microscope (although some exceptionally large species are visible to our naked eye, such as Thiomargarita namibiensis, which lives by accumulating spheres of elemental sulphur in its elongate cells in shelf sediments offshore Namibia in southwest Africa).

For what they lack in size, microorganisms make up for by their ubiquitous multitudes: 10 to 1000 billion in every teaspoon of soil, a billion or so in every 20 drops (one millilitre) of water, and each of us hosts on the order of 40,000 billion microbes thriving inside our body and out. Microbes have evolved to eke out a living in nearly every habitat on Earth, even those we don’t normally associate with the living world: kilometres below the surface in deep rock fractures, and deep below the surface of massive ice sheets. Microbes can even survive in the vacuum of space, and include those left behind on the Moon by astronauts (most likely in a dormant, resting state). We tend to consider microbes as dangerous and something to kill off indiscriminately with antibiotics and disinfectants. Although some are pathogens, most microbes are benign and many others beneficial, if not essential to our well-being. We are only beginning to appreciate the roles played by the highly diverse microbes that thrive in our gut, our mouths and in every nook and cranny of our skin.

Staphylococcus_aureus_VISA_2

The bacteria Staphylococcus aureus (colourised from CDC/Matthew J. Arduino, DRPH/Photo Credit: Janice Haney Carr, scale bar is 1 micrometre).

e_coli

Escherichia coli (E. coli) bacteria (credit: NIAID, each around 2 micrometres long)

Microorganisms are unicellular, with each individual consisting of a single cell whose features are either relatively simple or complex. Those with generally small, simple cells are referred to as prokaryotes and include two main groups: bacteria and archaea. Their simple cells contain a single, circular loose strand of DNA along with ribosomes housed within a cell wall and membrane. Those with generally bigger and more complex cells in which the DNA occurs tightly coiled within a separate nucleus enclosed by a membrane and, along with ribosomes, contain organelle structures are referred to as eukaryotes. The eukaryotes are thought to have evolved from the merging of different prokaryotes. For example, mitochondria organelles, which act as the energy furnace within eukaryotes, were once separate bacteria that were at some stage taken in by and subsumed by archaea. Bacteria and archaea evolved by around 3.5 billion years ago, while the eukaryotes only evolved later by around 2 to 1.5 billion years ago.  The evolution of eukaryotes was critical to the eventual evolution of all the big, visible organisms like plants and animals whose many cells are eukaryotic.

Ch1_p21_iii

Even magnified under a microscope, the many different species of bacteria and archaea can be difficult to tell apart. Some species have many tiny hair-like cilia or long thread-like tails (flagella) to whip them about (see photo top from http://www.cdc.gov/ecoli/), while others can bind together to form filaments or thin-film colonies. But most individual cells are either spherical to elongate in shape and difficult to tell apart from outward appearances. And this partially explains why archaea were only recently appreciated as being different from bacteria. One way to tell them apart is by what they do chemically, for bacteria and archaea are extremely versatile in the ways in which they metabolise. Some reduce carbon dioxide to methane, others fix nitrogen into nitrate while others convert nitrate to nitrogen, some oxidize sulphur to sulphate while others reduce sulphate to sulphide, and so on…each occupying the chemical niche best suited to their way of life. In essence, bacteria and archaea run most global elemental cycles at Earth’s surface, accelerating the pace of chemical reactions so vital to all life forms big and small. Just how many species there are remains largely unknown, but the application of genetics has facilitated our ability to identify them, with thousands of species typically analysed within any given soil or water sample.

Ch1_p24_iv

No sex please, we are prokaryotes! Bacteria and archaea reproduce by binary fission, where an individual makes an identical copy of itself and splits into two – a parent and its clone. They can also take up bits of DNA of interest that happen to be floating about in their surroundings or they can transfer specific bits of DNA among themselves. Such horizontal or lateral transfer of DNA helps them adapt to changes and to survive stressful times. They can also shut down and stay holed up as microbial cysts until better conditions return, even if that means remaining dormant for centuries or perhaps millions of years. Although microbes living today are probably not terribly different from the first of their kind that appeared 3.5 billion years ago, they can evolve rapidly. For example, so-called ‘superbugs’ evolve in hospitals by surviving the continuous onslaught of antibiotics and disinfectants.

Being the first to live on Earth, bacteria and archaea form the two deepest domains of life, and they are the life forms most likely to endure. Their small size, widespread distribution, genetic diversity and ability to shutdown make them far more likely than other organisms to survive whatever future catastrophes come along. As long as Earth is capable of supporting life it will always include a microbial world. In this sense, the meek shall inherit the earth, if the meekest among us are the smallest and simplest microorganisms. And if life exists on other Earth-like rocky planets orbiting within the wet and warm habitable zone of their stars, it too will most likely be microbial. When considering the abundance and amazing chemical promiscuity demonstrated by the element carbon on Earth, it seems reasonable to conjecture that microbes on other rocky planets are likely to be similar in many respects to our own. It is partly for this reason that we must take every precaution not to contaminate other worlds if we hope to determine unambiguously if life exists elsewhere.

©John S. Compton (www.johnscompton.com)       Earthspun logotan