Saturday 15 June 2024

Creationism in Crisis - How The Human Heart Evolved As Humans Adapted To An Active Life-Style

Study on architecture of heart offers new understanding of human evolution - Swansea University

According to research by an international team from Swansea University, Swansea, Glamorgan, Wales and the University of British Columbia, Okanagan, Canada, there is evidence that the human heart has evolved to facilitate the more active life-style of humans compared to their closest relatives, as they diversified.

Humans typically have a much more active life-style with a higher resting metabolic rate compared to chimpanzees. We also have larger brains. As hunter-gatherers, we typically walked and ran far more often and for much longer than do the other apes and we needed to lose the excess body heat this activity generated, so we needed a more hemodynamic heart able to meet those needs, which meant adaptations to changes in the twisting of the left ventricle (LV) - the chamber which pumps oxygenated blood around the body - during systole, and this meant a change in the degree of trabeculation inside the LV.

In the context of heart ventricles, what are trabeculae and what is their function? Trabeculae (also known as trabeculae carneae) are irregular ridges of muscle found on the inner walls of the ventricles of the heart. They play several important roles in the function of the heart:
  1. Structural Support: Trabeculae carneae help reinforce the ventricular walls. Their irregular structure provides additional strength and prevents the walls from sticking together during contraction, thereby maintaining the heart's shape and integrity.
  2. Facilitation of Blood Flow: The trabeculae carneae create a more turbulent flow of blood within the ventricles, which can help with the efficient mixing of blood and ensure a more thorough contraction and expulsion of blood from the ventricles during systole.
  3. Conduction System: Some trabeculae carneae are involved in the conduction system of the heart. For example, the moderator band (a type of trabecula) in the right ventricle contains part of the right bundle branch of the bundle of His. This helps coordinate the contraction of the right ventricle by ensuring that the electrical impulses are transmitted efficiently.
  4. Reduction of Blood Stagnation: By creating a complex surface within the ventricles, trabeculae carneae reduce the likelihood of blood stagnation, which can prevent clot formation and improve overall blood flow dynamics.

In summary, trabeculae carneae serve to strengthen the ventricular walls, enhance blood flow, support the heart's electrical conduction system, and reduce the risk of blood clots by preventing blood from pooling in the ventricles.

What is meant by 'ventricular twist'?

'Ventricular twist' refers to the complex motion of the heart's ventricles during the cardiac cycle, particularly during the contraction (systole) and relaxation (diastole) phases. This twisting motion is also known as 'ventricular torsion' or 'ventricular rotation.' Here’s a more detailed explanation:

Mechanics of Ventricular Twist
  1. Twisting Motion During Systole: During systole, the ventricles contract to pump blood out of the heart. The base (the top part of the ventricles, near the atria) of the heart rotates in a counterclockwise direction (when viewed from the apex), while the apex (the bottom tip of the heart) rotates in a clockwise direction. This results in a wringing motion, similar to wringing out a wet towel.
  2. Untwisting Motion During Diastole: During diastole, the ventricles relax and fill with blood. The previously twisted ventricles now untwist, helping to create a suction effect that aids in the efficient filling of the ventricles with blood.

Significance of Ventricular Twist
  1. Efficient Blood Ejection: The twisting motion enhances the efficiency of the heart’s pumping action. By adding a rotational component to the contraction, the heart can expel blood more forcefully and completely.
  2. Diastolic Suction: The untwisting or recoil of the ventricles during diastole contributes to the rapid filling phase. This elastic recoil generates a negative pressure that helps draw blood into the ventricles more efficiently.
  3. Mechanical Synchrony: The coordinated twisting and untwisting ensure that the contraction and relaxation phases are well-synchronized, promoting optimal cardiac function and maintaining a steady and efficient blood flow throughout the body.
  4. Clinical Relevance: Abnormalities in ventricular twist mechanics can be indicative of various cardiac pathologies, such as heart failure, myocardial infarction, or cardiomyopathies. Therefore, measuring and analyzing ventricular twist can provide valuable diagnostic and prognostic information.

Measurement of Ventricular Twist
Ventricular twist can be assessed using advanced imaging techniques such as speckle-tracking echocardiography (STE) or cardiac magnetic resonance imaging (MRI). These techniques allow for detailed visualization and quantification of the rotational mechanics of the heart.

In summary, ventricular twist is a crucial aspect of the heart’s mechanics, contributing to the efficient ejection and filling of blood in the ventricles. It is essential for maintaining optimal cardiac function and can be an important parameter in the assessment of heart health.
The team has shown there is a negative correlation between the degree of trabeculation and LV systolic twist. The inner wall of the LV of a healthy human heart is comparatively smooth. Their results are published in the journal Communications Biology and explained in a Swansea University News release:
An international research team from Swansea University and UBC Okanagan (UBCO) has uncovered a new insight into human evolution by comparing humans’ hearts with those of other great apes.

Despite humans and non-human great apes having a common ancestor, the former has evolved larger brains and the ability to walk or run upright on two feet to travel long distances, likely to hunt.

Now, through a new comparative study of the form and function of the heart, published in Communications Biology, researchers believe they have discovered another piece of the evolutionary puzzle.

The team compared the human heart with those of our closest evolutionary relatives, including chimpanzees, orangutans, gorillas, and bonobos cared for at wildlife sanctuaries in Africa and zoos throughout Europe.

During these great apes' routine veterinary procedures, the team used echocardiography—a cardiac ultrasound—to produce images of the left ventricle, the chamber of the heart that pumps blood around the body. Within the non-human great ape's left ventricle, bundles of muscle extend into the chamber, called trabeculations.

The left ventricle of a healthy human is relatively smooth, with predominantly compact muscle compared to the more trabeculated, mesh-like network in the non-human great apes. The difference is most pronounced at the apex, the bottom of the heart, where we found approximately four times the trabeculation in non-human great apes compared to humans.

Bryony Curry, first author
Centre for Heart, Lung and Vascular Health
School of Health and Exercise Sciences
University of British Columbia, Kelowna, BC, Canada.

The team also measured the heart's movement and velocities using speckle-tracking echocardiography, an imaging technique that traces the pattern of the cardiac muscle as it contracts and relaxes.

We found that the degree of trabeculation in the heart was related to the amount of deformation, rotation and twist. In other words, in humans, who have the least trabeculation, we observed comparatively greater cardiac function. This finding supports our hypothesis that the human heart may have evolved away from the structure of other non-human great apes to meet the higher demands of humans’ unique ecological niche.

Bryony Curry.

A human’s larger brain and greater physical activity compared to other great apes can also be linked to higher metabolic demand, which requires a heart that can pump a greater volume of blood to the body.

Similarly, Higher blood flow contributes to humans’ ability to cool down, as blood vessels close to the skin dilate—observed as flushing of the skin—and lose heat to the air.

In evolutionary terms, our findings may suggest selective pressure was placed on the human heart to adapt to meet the demands of walking upright and managing thermal stress.

What remains unclear is how the more trabeculated hearts of non-human great apes may be adaptive to their own ecological niches. Perhaps it’s a remaining structure of the ancestral heart, though, in nature, form most often serves a function.

Dr Aimee Drane, co-corresponding author
International Primate Heart Project
Cardiff Metropolitan University, Cardiff, UK
And Faculty of Medicine
Health and Life Sciences
Swansea University, Swansea, UK

The research team is grateful to the staff and volunteers who care for the animals in the study, including the teams at Tchimpounga Wildlife Sanctuary (Congo), Chimfunshi Wildlife Sanctuary (Zambia), Tacugama Chimpanzee Sanctuary (Sierra Leone), Nyaru Menteng Orangutan Rescue and Rehabilitation Center (Borneo), the Zoological Society of London (UK), Paignton Zoo (UK), Bristol Zoo Gardens (UK), Burgers’ Zoo (Netherlands) and Wilhelma Zoo (Germany).
Although the gross morphology of the heart is conserved across mammals, subtle interspecific variations exist in the cardiac phenotype, which may reflect evolutionary divergence among closely-related species. Here, we compare the left ventricle (LV) across all extant members of the Hominidae taxon, using 2D echocardiography, to gain insight into the evolution of the human heart. We present compelling evidence that the human LV has diverged away from a more trabeculated phenotype present in all other great apes, towards a ventricular wall with proportionally greater compact myocardium, which was corroborated by post-mortem chimpanzee (Pan troglodytes) hearts. Speckle-tracking echocardiographic analyses identified a negative curvilinear relationship between the degree of trabeculation and LV systolic twist, revealing lower rotational mechanics in the trabeculated non-human great ape LV. This divergent evolution of the human heart may have facilitated the augmentation of cardiac output to support the metabolic and thermoregulatory demands of the human ecological niche.

Mammals are a remarkably diverse class of vertebrates, capable of inhabiting every major biome on the planet. This diversity is associated with a vast range of environmental stressors and interspecific differences in posture and locomotion, creating very different hemodynamic challenges. Despite this remarkable diversity, the gross structure of the mammalian heart is highly conserved across species; retaining four chambers and a complete interatrial and interventricular septum1.

Although the gross structure of the mammalian heart is conserved, interspecific features exist. For example, heart shape varies considerably across species, from broad and flat in whales to long and narrow in terrestrial ungulates2. Variation in the cardiac phenotype is also present among closely-related mammals2, indicative of evolutionary divergence. While comprehensive data examining cardiac structure and function across the entire Hominidae taxon do not exist, preliminary work suggests that the left ventricle (LV) of adult male chimpanzees (Pan troglodytes) may be morphologically distinct from that of humans3. Prominent myocardial trabeculations, characterized by protrusions of the endocardium into the LV cavity with intertrabecular recesses, were previously observed in adult male chimpanzees3. This trabeculated phenotype differs from the relatively smoother ventricular wall typically observed in healthy humans4, suggesting that there may have been species-specific selective pressures on the heart during the evolution of Hominidae3.

Cardiac morphology and function are closely linked5; therefore, the discrete structural attributes of the chimpanzee and human LV likely coincide with differences in systolic and diastolic ventricular function. Such interspecific cardiac phenotypes may be the result of selection for the hemodynamic demands associated with each species’ ecological niche (i.e., the habitat and the role a species plays within an ecosystem). Indeed, previous data has shown that resting metabolic rate6, physical activity and daily locomotion7 are far greater in humans in comparison with other great apes, and so it is not surprising that cardiac output is also comparatively higher in humans3. The larger cardiac output in humans is likely supported by comparatively greater LV systolic and diastolic function (e.g., myocardial rotation and deformation), including LV twist3. LV twist, which is dependent upon the helical angulation of the aggregated cardiomyocytes8,9,10, is characterized by counter-directional rotation of the LV base and apex during systole. Together with the velocity of LV untwisting during diastole, LV twist helps facilitate efficient filling and ejection of the ventricle, especially during periods of heightened metabolic and thermoregulatory demand11,12.

The functional advantages associated with a LV capable of greater twist and untwisting velocity, combined with the preliminary data in adult male chimpanzees3, prompt the hypothesis that the human heart has diverged from a trabeculated ancestral phenotype to support the specific metabolic and thermoregulatory demands of the human niche. To test this hypothesis, we compared LV structure across all extant great apes using 2D echocardiography and further explored trabeculation in a subset of post-mortem chimpanzee (Pan troglodytes) hearts. We then compared LV rotation and deformation between human and non-human great apes to explore whether the trabeculated phenotype is associated with differences in LV systolic and diastolic functional mechanics. Our findings point to evolutionary divergence of the human LV away from the phenotype of all other non-human great apes, which may have had important implications for cardiac function in early humans.

Discussion (part of)


Collectively, the findings of this study support evolutionary divergence of the human LV away from a trabeculated ancestral phenotype, towards a ventricular wall with a proportionately greater compact myocardium. We propose that this adaptive evolution occurred to support the requirements of the human ecological niche, including an augmented cardiac output to facilitate sustained bipedal physical activity, a larger brain, and the associated metabolic and thermoregulatory demands.
Fig. 1: Comparison of left ventricular trabeculation in great apes.
The bullseye plots represent the trabecular:compact (T:C) ratio for each segment of the left ventricle. The outer layer of the bullseye plots represents the basal segments, the middle and innermost layers represent the midpapillary and apical segments of the left ventricle, respectively. Red segments correspond to an average T:C ratio of >2; orange segments correspond to an average T:C ratio >1.5–2; yellow segments correspond to an average T:C ratio of >1–1.5; green segments correspond to an average T:C ratio of >0.5–1; blue segments correspond to an average T:C ratio of <0.05. Echocardiographic images of the parasternal short-axis at the apex are shown at end-diastole. *No data were available for the basal or midpapillary segments in the orangutans due to artifact from laryngeal air sacs. †These T:C ratios compare favorably with other reports in healthy human cohorts, ranging from 0.2 to 0.965,66. ‡Anatomical labels have been provided in accordance with the conventional guidelines for cardiac chamber quantification by the American Society of Echocardiography and European Association of Cardiovascular Imaging52. However, we note that this clinical convention does not align with the recognized anatomical approach and may result in confusion across disciplines—see ref. 67 for further clarification.

Fig. 2: Graphical representation of the trabecular:compact (T:C) ratio for each segment of the left ventricle in chimpanzees.
The outer layer of the bullseye plots represents the basal segments, the middle and innermost layers represent the midpapillary and apical segments of the left ventricle, respectively. Red segments correspond to an average T:C ratio of >2; orange segments correspond to an average T:C ratio >1.5–2; yellow segments correspond to an average T:C ratio of >1–1.5; green segments correspond to an average T:C ratio of >0.5–1; blue segments correspond to an average T:C ratio of <0.05. Infant age class includes individuals of ≤4 years of age, juveniles between 5–7 years of age, sub-adults between 8-11 years of age and adults ≥12 years of age.

Fig. 3: Comparison of left ventricular morphology and mechanical indices of ventricular function between chimpanzees and humans.
Shortening along the long axis of the left ventricle (i.e., longitudinal strain) and deformation at the apex (i.e., apical circumferential and apical radial strain) were averaged across a mixed-sex, adult cohort of chimpanzees (n = 136) and represented in maroon. Blue reflects the deformation patterns of a mixed-sex, adult human cohort (n = 34). Dashed line represents aortic valve closure. Gray shading to the left of dashed line represents systole and white represents diastole.

Fig. 4: Relationship between markers of left ventricular (LV) function and apical trabeculation in the extant Hominidae taxon.
a Peak LV systolic apical rotation, shown in red, in a mixed-sex, adult cohort of humans (male n = 18, female n = 16), chimpanzees (male n = 59, female n = 51), bonobos (male n = 2, female n = 4), gorillas (male n = 4, female n = 6) and orangutans (male n = 10, female n = 6). b Peak LV systolic twist, shown in green, and (c) peak diastolic untwisting velocity, shown in blue, in a cohort of humans (male n = 18, female n = 16), chimpanzees (male n = 47, female n = 43), bonobos (male n = 1, female n = 3) and gorillas (male n = 4, female n = 5). Analyses of LV twist and untwisting velocity were not possible in all individuals, nor any of the orangutans due to artifacts from laryngeal air sacs, hence the reduced sample size. The exponential plateau curve is shown, with the 95% confidence bands represented by the dotted line. The mean and standard error are shown in black for each species.

Curry, B.A., Drane, A.L., Atencia, R. et al.
Left ventricular trabeculation in Hominidae: divergence of the human cardiac phenotype. Commun Biol 7, 682 (2024).

Copyright: © 2024 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
No crumbs of comfort there for creationists as the authors attribute everything to evolution by natural selection as humans diverged from the other great apes, nor is there any hint of a doubt that such an evolutionary divergence occurred.

The interesting thing from a biologists point of view is how the changes to the human heart reflect changes in our life-style as we adopted an upright gait and a hunter-gatherer life-style, necessitating running and walking long distances with additional demands on our heart to cope with the additional oxygen required and to dissipate the excess heat these activities generated, in addition to the increased demands our large brains were already imposing on our circulatory and thermoregulatory systems.

More work is now needed to understand whether the retention of this degree of trabeculation in the other apes has a benefit or whether it has simply been retained from a common ancestor. If the former, what evolutionary trade-off has there been for humans in losing these benefits?

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