Since the 1960s when the first hints of endogenous biomolecules in fossils began to appear, the idea has become increasingly hard to reject. Initially, these reports could easily be rejected as some kind of contamination.
However, evidence for it reached a crescendo a few years ago with the work of Mary Schweitzer and her team. Her research called into question the idea that soft tissue, protein and perhaps even DNA would naturally break down on the death of an animal. This idea was based on some solid reasoning - it seemed to always to happen thanks to the short the half lives of protein and DNA, and the presence of bacteria which would consume any organics around, following an organism’s death.
However, testing ideas that this was in fact so, and hence no soft tissue could ever be preserved, was limited, and often based on dubious presuppositions.
So, given the discovery of soft tissue, Schweitzer and her team needed to understand how it could plausibly be preserved for millions of years.
In this paper:-
- Schweitzer et al. describe how they found a potential answer to the problem.
The paper introduces the problem by describing the history leading up to its identification. That soft tissue structures could be preserved was found as early as the 1960s, but how this preservation could occur was never addressed by then existing models of fossilisation i.e. mineral replacement. With the identification of soft tissue structure, ideas were developed to explain its preservation such as “microbially mediated stabilization” which then went on to a process of secondary mineralization. The authors note that few of these preservation modes were actually tested experimentally.
Then, still-soft biomaterials began to be identified in fossils, presenting an additional problem to be solved. How could the molecules be preserved and yet remain soft?
Early explanations were that these purported soft tissues (and molecules) were little more than contaminants. However, Schweitzer et al. and others made convincing arguments that they were not contaminants but rather did belong to the fossils. Yet there were “hypothesized temporal limits on molecular preservation of less than 1 Myr for proteins and approximately 100000 years for DNA [26–30] (but see [31]) that are based upon degradation proxies of heat and/or pH [28,32], theore- tical models of breakdown kinetics [33,34], and, recently, extrapolation from a select and time-limited set of fossils [35].”
So the problem was to find a process of preservation in which “soft tissues and the proteins comprising them ... persist beyond these limits, a mode of preservation sufficiently rapid to outpace decay ...”
Schweitzer et al. proposed that iron from haemoglobin (in particular) in association with oxygen does in fact provide this stabilization.
They tested their idea on ostrich bones.
The paper describes why iron could be the key to stabilization, and how the source of it probably derives from the organism itself, just after death. Organisms do contain a lot of iron, and while an animal is alive, the iron needs to be bound up, lest it react unfavourably with other molecules important to the process of living. On death, naturally, iron does not need to be bound and its liberation from various molecules forms the basis of association with soft tissue preservation in fossils. It’s one thing the researchers noted, when soft tissue is found, iron seems to be there as well.
Once liberated, the iron, via the formation of “oxy-radicals”, facilitates “protein cross-linking [54] in a manner analogous to the actions of tissue fixatives (e.g. formaldehyde), thus increasing resistance of these ‘fixed’ biomolecules to enzymatic or microbial digestion [55,56].”
The researchers spend a section of their paper demonstrating the link between iron and soft tissues in the fossil bones as well as in their ostrich model. Then they describe their ostrich model, and the control they used to validate the model against:-
“Ostrich vessels were incubated in a concentrated solution of red blood cell lysate (see the electronic supplementary material) to approximate post-mortem erythrocyte lysis. Control tissues were prepared identically, then incubated in either sterile dis- tilled water or phosphate buffered saline (PBS).”
They stress the importance of haemoglobin:-
“Haemoglobin was chosen to test its preservation properties for four reasons: (i) HB is in known to be bacteriostatic [63,64]; (ii) in the presence of dioxygen, HB produces free radicals [65]; (iii) blood vessels fill with large amounts of HB after death as red cells begin to die and lyse, thus it is naturally present in large vertebrates [45]; and (iv) haeme released from HB, when degraded, will release iron, possibly accounting for the iron particles associated with preserved soft tissues [42,66] (figure 1).”
Schweitzer et al. then discuss their ostrich model.
They obtained ostrich soft tissue from an ostrich farm and set up a test model and a control. The test was of:-
“Ostrich vessels ... incubated in a concentrated solution of red blood cell lysate (see the electronic supplementary material) to approximate post-mortem erythrocyte lysis.”
- while the control was:-
“... prepared identically, then incubated in either sterile distilled water or phosphate buffered saline (PBS).”
Their figure 4 shows the differences between the results of the incubation of HB treated tissue, HB + O2 treated tissue and tissue incubated in water. The results are dramatic. They write:-
“HB-treated vessels have remained intact for more than 2 years at room temperature with virtually no change, while control tissues were significantly degraded within 3 days. Indicators of tissue stability include thick vessel walls (figure 4a,b, black arrows) and visible surface structures consistent with endothelial nuclei (figure 4a,b, white arrowheads). In many cases, material could be seen inside the vessel lumen, appear- ing most often as structureless masses (figure 4a,b, asterisk). There was no difference between tissues incubated in HB/ hypoxy and HB/oxy conditions (see the electronic supplementary material), including the presence of the intravascular material, except that distinct red blood cells were also present in the HB/oxy condition (figure 4c,d, asterisk)."
“The range of tissue stabilities observed with differing combinations of HB and O2 were: HB + O2 > HB - O2 > -O2 >> +O2, emphasizing the importance of both HB and oxygen to tissue stabilization.”
And later:-
“In our test model, incubation in HB increased ostrich vessel stability more than 240-fold, or more than 24 000% over control conditions."
While virtual stability over 2 years (at least) is nowhere near a few tens of millions of years, it certainly casts the conventional wisdom into doubt. They go on to discuss the results of their experiment including the role of iron in confounding efforts to sequence these biomolecules. Exactly how iron does what it does, is still a bit of a guess:-
“The iron may be directly protecting proteins by blocking active sites recognized by enzymes of degradation (supported by the increase in antibody signal after treatment with iron chelator), or it may be providing protection indirectly by binding to oxygen, and thus preventing oxidative damage [68,69] or outcompeting bacterial mechanisms, similar to ferritins [45].”
However, there is now hope that the identification of soft tissue in many ancient organisms way offer new ways of studying their remains.
However, evidence for it reached a crescendo a few years ago with the work of Mary Schweitzer and her team. Her research called into question the idea that soft tissue, protein and perhaps even DNA would naturally break down on the death of an animal. This idea was based on some solid reasoning - it seemed to always to happen thanks to the short the half lives of protein and DNA, and the presence of bacteria which would consume any organics around, following an organism’s death.
However, testing ideas that this was in fact so, and hence no soft tissue could ever be preserved, was limited, and often based on dubious presuppositions.
So, given the discovery of soft tissue, Schweitzer and her team needed to understand how it could plausibly be preserved for millions of years.
In this paper:-
Originally posted by reference inside
The paper introduces the problem by describing the history leading up to its identification. That soft tissue structures could be preserved was found as early as the 1960s, but how this preservation could occur was never addressed by then existing models of fossilisation i.e. mineral replacement. With the identification of soft tissue structure, ideas were developed to explain its preservation such as “microbially mediated stabilization” which then went on to a process of secondary mineralization. The authors note that few of these preservation modes were actually tested experimentally.
Then, still-soft biomaterials began to be identified in fossils, presenting an additional problem to be solved. How could the molecules be preserved and yet remain soft?
Early explanations were that these purported soft tissues (and molecules) were little more than contaminants. However, Schweitzer et al. and others made convincing arguments that they were not contaminants but rather did belong to the fossils. Yet there were “hypothesized temporal limits on molecular preservation of less than 1 Myr for proteins and approximately 100000 years for DNA [26–30] (but see [31]) that are based upon degradation proxies of heat and/or pH [28,32], theore- tical models of breakdown kinetics [33,34], and, recently, extrapolation from a select and time-limited set of fossils [35].”
So the problem was to find a process of preservation in which “soft tissues and the proteins comprising them ... persist beyond these limits, a mode of preservation sufficiently rapid to outpace decay ...”
Schweitzer et al. proposed that iron from haemoglobin (in particular) in association with oxygen does in fact provide this stabilization.
They tested their idea on ostrich bones.
The paper describes why iron could be the key to stabilization, and how the source of it probably derives from the organism itself, just after death. Organisms do contain a lot of iron, and while an animal is alive, the iron needs to be bound up, lest it react unfavourably with other molecules important to the process of living. On death, naturally, iron does not need to be bound and its liberation from various molecules forms the basis of association with soft tissue preservation in fossils. It’s one thing the researchers noted, when soft tissue is found, iron seems to be there as well.
Once liberated, the iron, via the formation of “oxy-radicals”, facilitates “protein cross-linking [54] in a manner analogous to the actions of tissue fixatives (e.g. formaldehyde), thus increasing resistance of these ‘fixed’ biomolecules to enzymatic or microbial digestion [55,56].”
The researchers spend a section of their paper demonstrating the link between iron and soft tissues in the fossil bones as well as in their ostrich model. Then they describe their ostrich model, and the control they used to validate the model against:-
“Ostrich vessels were incubated in a concentrated solution of red blood cell lysate (see the electronic supplementary material) to approximate post-mortem erythrocyte lysis. Control tissues were prepared identically, then incubated in either sterile dis- tilled water or phosphate buffered saline (PBS).”
They stress the importance of haemoglobin:-
“Haemoglobin was chosen to test its preservation properties for four reasons: (i) HB is in known to be bacteriostatic [63,64]; (ii) in the presence of dioxygen, HB produces free radicals [65]; (iii) blood vessels fill with large amounts of HB after death as red cells begin to die and lyse, thus it is naturally present in large vertebrates [45]; and (iv) haeme released from HB, when degraded, will release iron, possibly accounting for the iron particles associated with preserved soft tissues [42,66] (figure 1).”
Schweitzer et al. then discuss their ostrich model.
They obtained ostrich soft tissue from an ostrich farm and set up a test model and a control. The test was of:-
“Ostrich vessels ... incubated in a concentrated solution of red blood cell lysate (see the electronic supplementary material) to approximate post-mortem erythrocyte lysis.”
- while the control was:-
“... prepared identically, then incubated in either sterile distilled water or phosphate buffered saline (PBS).”
Their figure 4 shows the differences between the results of the incubation of HB treated tissue, HB + O2 treated tissue and tissue incubated in water. The results are dramatic. They write:-
“HB-treated vessels have remained intact for more than 2 years at room temperature with virtually no change, while control tissues were significantly degraded within 3 days. Indicators of tissue stability include thick vessel walls (figure 4a,b, black arrows) and visible surface structures consistent with endothelial nuclei (figure 4a,b, white arrowheads). In many cases, material could be seen inside the vessel lumen, appear- ing most often as structureless masses (figure 4a,b, asterisk). There was no difference between tissues incubated in HB/ hypoxy and HB/oxy conditions (see the electronic supplementary material), including the presence of the intravascular material, except that distinct red blood cells were also present in the HB/oxy condition (figure 4c,d, asterisk)."
“The range of tissue stabilities observed with differing combinations of HB and O2 were: HB + O2 > HB - O2 > -O2 >> +O2, emphasizing the importance of both HB and oxygen to tissue stabilization.”
And later:-
“In our test model, incubation in HB increased ostrich vessel stability more than 240-fold, or more than 24 000% over control conditions."
While virtual stability over 2 years (at least) is nowhere near a few tens of millions of years, it certainly casts the conventional wisdom into doubt. They go on to discuss the results of their experiment including the role of iron in confounding efforts to sequence these biomolecules. Exactly how iron does what it does, is still a bit of a guess:-
“The iron may be directly protecting proteins by blocking active sites recognized by enzymes of degradation (supported by the increase in antibody signal after treatment with iron chelator), or it may be providing protection indirectly by binding to oxygen, and thus preventing oxidative damage [68,69] or outcompeting bacterial mechanisms, similar to ferritins [45].”
However, there is now hope that the identification of soft tissue in many ancient organisms way offer new ways of studying their remains.
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