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Earthworms must be the most famous animal icon of soils. There are over 6000 earthworm species, but not found equally on all continents. Most earthworms dwell in soil, but some live in leaf litter, decaying logs and riverbanks.
I suspect most people would think earthworms would have made an appearance in soil before this. Darwin did not express an opinion on when he though they arrived. While there is some disagreement as to when they appeared, it certainly seems around this period. There is no evidence of them being here before. Yes, they don't make good fossils, but they do leave tracks and burrows.
They are the big earth movers and shakers and well-known for their physical powers. But they had would have to have something to move and shake. Perhaps, there wasn’t enough soil until this period. Their burrows, thanks to their mucus, were stronger to provide ways for other creatures - like insect larvae, These larvae are walking about rather squeezing through.
As well as their physical impacts, they would have had a massive impact on several existing chemical and biological soil processes, like secondary aggregation, and the third wave of decomposition, each by spreading humification.
Darwin's love of worms
Darwin's love of worms
His interest in earthworms never waned, but it was not until 1880 that he began to draw together the observations he had gathered to write a book on the subject. Soon worm excrement was trusted to postal services, and Darwin acquired casts from India and Australia . Darwin brought to the topic the sustained attention that characterised all his work, and by the end of the year admitted that ‘My whole soul is absorbed with worms just at present!’ (letter to W. T. Thiselton Dyer, 23 November [1880] (Royal Botanic Gardens, Kew)). In the end, however, as he told his son William, what he hoped his book would reveal is that ‘worms have much bigger souls than anyone wd suppose’ (letter to W. E. Darwin, 31 January [1881]
We think earthworms are absolutely crucial in turning soil over – called bioturbation. Darwin describes this in detail n his 'Vegetable Moulds and Earthworms. He was helped by various family members, collecting casts, digging dirt and even playing piano, which somehow proved worms cannot hear...
My theory
My theory
I believe that the great earthworm radiation was in this Jurassic period. Any theory needs evidence. But there are v few worm-like fossils. Luckily there is other evidence - of possible earlier relatives. There were enchytraeids which had been around in peat bogs for well over 100my. Although they are much smaller than earthworms, they belong to the same subclass of animals (Oligochaeta, which is found within the class Clitellata (and phylum Annelida ) and some of may have have bigger and stronger to burrow into soil, rather than just wriggle round. Or their ancestors may have been blackworms (Lumbricilidae) living in freshwater that came into the soil from there.
Worms make the world go round
- Clutterbuck
Earthworms are major terrestrial ecosystem engineers and their economic impact is immense—earthworms turn over, aerate and drain soils, providing crucial assistance to farmers and gardeners, and compost-dwelling species are used to process food waste and animal manures. Their own intestines they carry many microbes feeding on their mucous, which will be helping decomposition - probably both mineralisation and humification. But it is the spread of humification which coincides with their presence.
Darwin, worms & the plough
Darwin, worms & the plough
“The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.”
Charles Darwin, The formation of vegetable mould through the actions of worms, with observations on their habits, John Murry pg. 313
I think Darwin should have said 'burrowed' as we now see ploughing/tilling as breaking down soil structures
"Brinkhurst (1982) reorganized the oligo-chaete taxa in four orders, Haplotaxida (now containing only Haplotaxidae), Lumbriculida (as before, with the single family Lumbriculidae), Tubificida (the bulk of small aquatic oligochaetes plus the amphibious Enchytraeidae), and Lumbricida (all earthworms plus Moniligastrida and the only opisthopore aquatic family, the Alluroididae)...Modern authors usually ignore the order level " (Shmeltz et al 2021)
Origins
Origins
Earthworms have not been around from the beginning. These big ‘ecosystem engineers’ may well have came to the soil when it was already well-formed, so providing a useful new substrate for living. When and how did they arrive? Well, you’ll never guess but,
Most discussions about earthworm origins usually go back a long way before the one I'm interested in - from potworms to earthworms. Both belong to the big phylum called ‘ringed’ worms (Annelids), which all have a general body cavity or coelom. This enables them to have a gut with a beginning middle and end - instead of wastes going back through the mouth. There are 3 classes of Annelids, two predominantly marine, the other called Oligochaetes, which include virtually all earthworms and enchytraeids, and have less bristles and no appendages.
“Although earthworms are among the most familiar and economically important groups of large invertebrates, their evolutionary history is not well understood”. If we try to work out where worms come from in the distant past, how they got here, then we open the proverbial ‘can of worms’.
Can of worms
Can of worms
In years gone by, the dominant theory of the origin of earthworms was that they came from ancestors of jellyfish and anemones. These creatures do not have a coelom or body cavity and so are called ‘acoelomates’. However, a more recent theory points to evolution the other way round – that the jellyfish and anemones came from worms, making one of its research authors say “This is the most political fraught paper I’ve ever written." (Maxmen 2011)”
Conflicting evidence
Conflicting evidence
Worms do not make good fossils, but a trace fossil (like the dinosaur tracks) consisting of a convoluted burrow partly filled with small faecal pellets may be evidence that earthworms were present 250 million years ago. Body fossils go back to 260mya, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.
Worm fossils go back over 500mya, but ringed worm fossils only appeared around 300mya. Current evidence (Parry et al 2013) from molecular clocks and the fossil record suggest that the earliest annelids are around 500 mya, but oligochaetes only started radiating 250mya and spreading out in this period. Molecular phylogeny and divergence times analysis suggest that the divergence time of Moniligastridae (Outgroup) and Lumbricidae (Group 1) was 215.84 mya. I am curious how molecular analysis can be that accurate.
According to fossils, the earliest good evidence for oligochaetes (marine and earthworms) occurs as late as 66 million years ago. Early examples of their wider group - annelids - go back further. Other suggest that these animals evolved around the same time as flowering plants from 130 to 90 million years ago - in the early Cretaceous.
The name Protoscolex was given to a genus of segmented worms without bristles found in the Ordivician (450mya) of Kentucky, US. Another species, in the same genus, was found in Herefordshire, England, but it is unclear whether these worms are in fact oligochaetes. Stephenson's classic classification in 1930 has the common ancestor of oligochaetes as the aquatic family Lumbriculidae (blackworms).
Blackworm
Earthworms and their relatives lay their eggs in cocoons, and sometimes these cocoons fossilize. Cocoon fossils of leeches, another Annelid, are known from around 200 million years ago, which tells the minimum age of the common ancestor of leeches and earthworms. These fossils were used to calibrate the family tree and infer divergence dates.
The more advanced families such as Glossoscolecidae, Hormogastridae, and Microchaetidae may have evolved later, and I'm suggesting Lumbricidae, the earthworms that we recognise today, evolved in this period. Since we evolved, the Lumbricidae has followed humans round the world and displaced many native species of earthworm... There is - as usual - a debate re tectonics v birds dispersal as to how this may happen.
“The age of earthworm genera living on both sides of the Atlantic is inferred, based on the continental drift hypothesis, as equal or superior to the age of the ocean itself (about 180 My or more. As such it corresponds to the age of reptiles such as Plesiosaurus, Ichthyosaurus, and Tyrannosaurus rex. Similarly, the world-wide distribution of the current earthworms families is regarded as a mirror of their spread on the most recent supercontinent Pangea. Unfortunately, the continetal drift hypothesis cannot explain how the earthworm families achieved to be widely distributed already on Pangea.” (Pavlicek 2017)
Sac & Segments together
Sac & Segments together
It used to be thought that the segmented bodies of arthropods and annelids made them very close relatives with a fairly-recent common ancestor. But it mayy be the annelids are much more closely related to the non-segmented molluscs which have a coelom.
The way worms burrow relies on both a sac and segments working together to squeeze through soil. In Gee’s ‘Short history of life on earth’ (p25) he says perfectly: "If the segments are contained in a rough external tube of muscle, you can essentially force yourself into the soil by exerting pressure on it . And if you move round like that, then you are an earthworm" It is just that it was not to happen until 200 million years after the period he was talking about.
Sac or Segments first?
Sac or Segments first?
As usual there are massive debates about the lineages of annelids, particularly ‘the earlier complex of theories involving the simultaneous evolution of coelom and segmentation’. We’ve seen the coelom is a sac in the body to house the organs, common to all ringed worms, so the gut floats free wrapped with muscles. Worms are segmented repeating various body parts in each segment.
Molluscs have a reduced coelom, only round their heart, so it was not until the evolution of the well-developed coelom, that its full potential was realised.
In famous review from over 40 years ago, the ‘evolution of Annelida’ says: “concepts are in accord with recent locomotor theory separating the evolution of the coelom and segmentation as a two-step process related to sustained burrowing activity, as opposed to the earlier complex of theories involving the simultaneous evolution of coelom and segmentation. Unsegmented coelomates are seen as representatives of an intermediate condition between acoelomates and segmented coelomates instead of problem phyla derived by degeneration of segmented ancestor”(Brinkhurst 1982)
This means the body sac in worms to house their organs came first. Sometime after that, the segmentation, characteristic of these creatures, arrived. Both are needed both to burrow.
The segmented body plan is where body features are repeated in each body segment. It allows animals to become bigger by adding “compartments” while making their movement more efficient. Each segment contains structures for digestion, excretion, and locomotion.
The fluid within the body sac or coelom of each segment makes a rigid support system for the worm - similar to the rigidity of a filled water balloon. This rigidity in annelid segments creates a strong hydrostatic skeleton that muscles can push against.Each segment can move as a muscle and independently from the rest of the segments, and the worms move through a process of expansion and contraction.
The vital process for worms is to burrow through soil, making them the soil engineers. To do so clearly involves both these body features. Yet according to the locomotor theory, it would seem the sac came first. What would have been the environmental determinants of this? Is it possible that the evolution of the coelom enabled worms to eat, digest AND poo, but segmentation, giving more flexible strength against soil, came sometime later?
Divergence and worm evolution
Divergence and worm evolution
The ideas of a German zoologist, Johann Michaelsen in the early 1900s about earth-worms were seen as evidence for the then radical theory on plate tectonics. Since then, ecologists have also noted that latitude is the principal factor determining the presence of earthworm communities worldwide. "The main split is between the families of the Northern Hemisphere (Lumbricidae, Homogastridae and Criodillidae) and the Southern hemisphere (Megascolecidae, Microchaetidae, Rhinodrilidae, Almidae, Glossoscolecidae and Eudrilidae). The date of this split correlates with the break-up of Pangea 175 Mya"(Straleen 2021)
North & South
North & South
The earthworm family tree consists of two major branches, both with subgroups in the Northern and Southern Hemispheres. One branch has families in eastern North America and in Madagascar. The other branch contains the vast majority of earthworm species. The northern subgroup includes Lumbricidae, comprising nearly all familiar European species, and the southern subgroup includes Megascolecoidea.
Is the broad geographic distribution of earthworms Southern and Northern hemispheres due to dispersal between continents (e.g., by rafting) or vicariance—riding the continents. In the Jurassic period the continents were not that far apart. The continents have drifted apart over several hundred million years. Some researchers have tried to use their family tree, calibrated to geological time, to assess the relative importance of these competing mechanisms.
Their analyses concur with the ancestor of all living earthworms probably lived 200mya ago - about as old as mammals and dinosaurs. Estimates for the divergences between the Northern and Southern Hemisphere subgroups of both branches of earthworms again fall between 178-186 million years ago, again coinciding with the breakup of the supercontinent Pangaea 180-200 million years ago.
“This corroborates the hypothesis that continental breakup influenced early earthworm diversification. This also implies that earthworms likely inhabited Antarctica before the continent’s southward drift made it inimical to most terrestrial animal life. This phylogeny(family tree) also provides a robust framework for investigating several questions about earthworm evolution, one in particular. Earthworms have transitioned from terrestrial to aquatic habitats and vice versa. Most species in Clitellata (the group that includes earthworms) are aquatic, so earthworm genomes may retain ancestral genes that enable transitions between habitats. Alternatively, certain features may have been reinvented at each habitat transition. We have begun to explore this question in our future work.” So, that hasn’t quite put the lid back on the ‘can of worms’.
Molecular
Molecular
Given the problems with worm fossils, perhaps the molecules may come to the rescue. A study by USNS put earliest at 209mya although another has the ancestor a bit later - but clearer.
US National Science Foundation
US National Science Foundation
“An US National Science Foundation to study annelid phylogeny (family tree), we harnessed high-throughput DNA sequencing to generate transcriptomes for representatives of nearly all of the eighteen living earthworm families as well as several groups thought to be closely related to them and used these data to infer phylogenetic relationships.”
Our analyses reveal that the ancestor of all living earthworms probably lived over 209 million years ago, making earthworms about as old as mammals and dinosaurs. Our date estimates for the divergences between the Northern and Southern Hemisphere subgroups of the two major branches of earthworms fall between 178-186 million years ago, coinciding with the breakup of the supercontinent Pangaea 180-200 million years ago and corroborating the hypothesis that continental breakup influenced early earthworm diversification. This also implies that earthworms likely inhabited Antarctica before the continent’s southward drift made it inimical to most terrestrial animal life." (Anderson et al 2014)
Later candidate
Later candidate
Another study had the worm ancestor a bit later, but a clearer candidate for it.
“Our chronogram (a family tree where branches are in units of time) suggests that lumbricids emerged in the Lower Cretaceous (the next period) in the Holarctic region and that their diversification has been driven by tectonic processes (e.g., Laurasia split) and geographical isolation. Our chronogram and character reconstruction analysis reveal that spermathecae number does not follow a gradual pattern of reduction and that parthenogenesis arose from sexual relatives multiple times in the group; the same analysis also indicates that both epigeic and anecic earthworms evolved from endogeic ancestors. These findings emphasize the strong and multiple changes to which morphological and ecological characters are subjected, challenging the hypothesis of character stasis in Lumbricidae" (Dominguez et al 2015) ”
The first worms to arrive were endogeic (deep horizontal burrowing) worms who tunnelled into soils like never before, and in so doing added the 3rd wave of decomposition - humification. The humic substances had new sticking properties to build secondary aggregates enabling new soil structures - and even more spaces for them to live..
Endogeic Entry
Endogeic Entry
The first earthworms burrowed horizontally below the surface (Endogeic). Their tunnels would They gave rise to those on the surface (Epigeic) and those that burrow straight down (Anecic). So how did the endogeics get there?
Could they have evolved from Enchytraeids (potworms)? “The Enchytraeidae are typically 10–20 mm in length and they are anatomically similar to the earthworms, except for the miniaturization and rearrangement of features overall…. Sexual reproduction in enchytraeids is hermaphroditic and functions similarly to that in earthworms."
Or it could be blackworms, that live in freshwater. it is conceivable that the ancestors were blackworms (Lumbricilidae) that could have come from water into soil banks.
Analysis indicates that both epigeic and anecic earthworms evolved from endogeic ancestors. These findings emphasize the strong and multiple changes to which morphological and ecological characters are subjected, challenging the hypothesis of character stasis in Lumbricidae" (Dominguez et al 2015) ”
From 'deep horizontal' to 'vertical' and 'surface' dwellers
From 'deep horizontal' to 'vertical' and 'surface' dwellers
I believe that the can of worms could be closed if we see that potworms or blackworms were around for at least a hundred million years before these earthworms. They may well have been major contributor in the construction of soil structures, able to move through other's tunnels, where they chewed and pooed their way through soil, making larger aggregates for even bigger tunnels a nd inhabitants. It is possible to see that with the growth of soil, deeper and more extensive, more segmentation and muscle-building helped move the heavy soil. They get bigger to become the worms we know as endogeic – burrowing below the surface of the soil, which is now strong enough to support itself and thus not squash these wrigglies.
In the next period we will see how these worms evolved in the soil, some preferring the surface (Epigeic) and those that went more vertically up and deeper down (Anecic)
Bioturbation
Bioturbation
Endogeic earthworms live within the soil and utilize it for sustenance. Crucially, they burrow into the soil in a horizontal fashion preferring to remain within a given stratum as they move around in search of nutrients. To a limited extent, endogeic earthworms can reuse their own burrows. In stark contrast to epigeic earthworms, endogeic earthworms tend to possess a pale complexion, exhibiting such colours as grey, pink, green or blue.
So, what changes would this have made to soil? These are the most significant soil changes for over a couple of hundred million years and have lasted as long after. Now, on top of the existing soil, the leaves and other fallen organisms and creatures would meet up with others existing in the soil. This bioturbation and soil mixing, which we now take for granted, takes a couple of years nowadays for the leaves and surface soil to mix. We know that worm move and mix mountains, but what about their impacts on biological and chemical processes?
Anecic worms (on rt early followed by epigeic worms on surface and, after the moo, endogeic worms along bottom
Mucus
Mucus
Their mucus smooths their burrowing. Earthworm mucus is vital for the worms to move through the heavy-duty soil. The drag-reducing characteristics of earthworm epidermal mucus has inspired the development of some potential bionic applications with lubricating functions for soil-tillage implements. (Zhang et al 2016)
That mucus, which would also be in their poo, consists of proteins and carbohydrates, which are adsorbed to clay and iron particles making for more aggregation. Different worms make different aggregates. The volume ratio of mineral grains within the aggregates is significantly different according to earthworm species“ (Le Bayon et al 2020)
‘Tomography’
-images inside the body
‘Tomography’
-images inside the body
Soil structure is closely linked to biological activities. X‐ray micro computed ‘tomography’ (to create detailed images of the inside of the body) could be a useful tool to discriminate soil aggregates. It may help work out their origin and their formation processes for a better comprehension of soil structure properties and genesis. One study aimed to determine different X‐ray parameters for differentiating earthworm casts belowground from non‐earthworm aggregates, and to evaluate if these parameters can also serve as specific “tomographic signatures” for the studied earthworm species.
Three species, representing the different groups, of earthworms were tested separately: the epigeic Lumbricusrubellus, the anecic Lumbricusterrestris and the endogeic Allolobophorachlorotica. The X‐ray helped distinguish earthworm aggregates from non‐earthworm ones. “The volume ratio of mineral grains within the aggregates is significantly different according to earthworm species. So, X‐ray μCT is a powerful and promising tool for studying the composition of earthworm casts and their formation. However, future research is needed to take into account the shapes and spatial distribution of the aggregates' components, in particular the different states of organic matter decomposition."(Le Bayon et al 2020)
Driloshere
Driloshere
The Earthworms’ nutrient rich mucus, release bacteria within casts and translocate litter into the subsurface. They not only shape the structure of soils but also the chemical milieu of the ‘drilosphere’ where mucus forms a prominent fraction of Organic Matter. The drilosphere is the part of the soil influenced by earthworm secretions, burrowing and castings, and the soil which has gone through the worm gut. The average thickness of the drilosphere is 2 mm, but it can be much wider (about 8 mm) of litter-feeding earthworms.
Mucus & minerals
Mucus & minerals
Mucus from anecic (Lumbricusterrestris L.) and endogeic (Aporrectodeacaliginosa Sav.) earthworm species consists of proteins and carbohydrates, which is adsorbed to clay and iron particles. “We conclude that the specific adsorption of earthworm mucus constituents to soil minerals leads to the formation of mucus-mineral associations. These associations contribute to retention of organic substances from earthworm mucus in soil (micro—aggregates) and explain the altered physicochemical properties of earthworm-formed aggregates." (Guhra et al 2020) The biogenic extracellular polymeric substances (EPS) are known to form organo-mineral associations, to encourage further attachment into soil aggregates.
Microbes
Microbes
Earthworms provide a more anaerobic environment in their guts compared with the soil outside their bodies. “ results reinforce the general concept that the earthworm gut is not microbiologically equivalent to soil and also suggest that the earthworm gut might constitute a microhabitat enriched in microbes capable of anaerobic growth and activity.” Karsten (1995)
Inside
Inside
Earthworms decompose organic matter by having microbial communities that inhabit their digestive track or the structures they build. These in turn contribute to make up the drilosphere (above), a hotspot for microbial activity.
Mutualism
Mutualism
Many of the microbes that pass through the earthworm gut appear to be transient and are simply eaten by the earthworm as a means to acquire nutrients. However, some microbes react well to the anoxic, moist environment they encounter inside the digestive tract and become quite active. This results in a complex mutualism, where communities have altered composition after digestion due to the selection for those capable of anaerobic metabolism [2]. Members of the Acidovorax genus are thought to degrade proteins, allowing the host earthworm to reabsorb nitrogenous compounds that would otherwise be excreted [10]. Some earthworm gut-associated microbes, such as Streptomyces, are known to produce cellulases, which would help the earthworm host to degrade plant residues, while other gut microbes, like Mycobacterium utilize common soil components, such as humic acids (see Thakuria 2009 for further discussion on the ecological roles of earthworm gut microbes). It is well kown that earthworms decompose dead matter, but lets look a bit closer at the two main decomposing processes.
Outside
Outside
“Specific bacterial groups consistently increase in soils where earthworms are present, regardless of the earthworm functional group. The extent of this increase seems to be dependent upon the type of substrate under study. Our synthesis also reveals that endogeic and anecic earthworms regularly induce an increase in soil nutrients, whilst this positive effect is not as evident in the presence of epigeic earthworms…(but unable) to prove a direct causal relationship between specific signal molecules, earthworms and plant growth promotion" (Suaza et al 2019)
How earthworms modify the structure of soil microbial and root communities, and their associated processes, still needs exploring...
Mineralisation
Mineralisation
One of the key soil processes is the ‘mineralisation’ of organic compounds, to produce minerals like nitrates and phosphates for plant uptake. The role of earthworms is mixed. “N mineralised by earthworms and their associated microorganisms might be used more readily by plants, thereby masking an increase in soil available N concentrations. Similarly, the amount of readily available phosphorus (P) has been shown to be affected by earthworms, levels of available P being higher in casts or in biopores formed by L. terrestris than in the bulk soil" (Medina-Sauza et al 2019) Water-extractable P can be 30-1000X more in casts than bulk soil, and these ‘P hotspots’ are more prolific in epigeic species.
Mycorrhiza
Mycorrhiza
Again worms seem to have mixed response. Over 60% of the reviewed cases reported that earthworm activities enhanced root colonization by mycorrhizal fungi while 25% reported a reduction in root colonization. There is quite a variation in studies of fungal spores in soil digested by earthworms, in particular between the 3 different functional groups. “For anecic and epigeic earthworms the ratio of positive to negative and neutral effects on root colonization by mycorrhizal fungi worked out to 3:1 and 4:1, respectively, for endogeic species this was inverted to a 1:2 ratio. The mechanisms that may differentially affect root colonization by mycorrhizal fungi when interacting with anecic/epigeic, and endogeic earthworms are poorly understood." ((Medina-Sauza et al 2019) Only a handful of species of fungi have been used in the experimentation, so more research is needed to understand this relation better, as it could be very important.
Earthworms, often seen as the heavy engineers of soils arrived in this period, as the soil deepened and darkened. Soil got deeper because ectomycorrhizal fungi could penetrate into mineral soils helping to make roots go deeper - and hence trees grow bigger. Along come the ubiquitous oribatid mites to chew and poo their way through much more detritus, often left by the earthworms after breaking down the big bits. Earthworms spread out in the next period, as we will see. Worms 145-66mya
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