Creationism in Crisis
Scientists Have Discovered How Plants Evolved About 420 Million Years Ago
Scientists Have Discovered How Plants Evolved About 420 Million Years Ago
Selaginella
Photo: Vicky Spencer
Saginella root tip
Photo: Jill Harrison
News today of yet another casual and incidental refutation of some of the Creation cult's fundamentals.
First, a little anecdote from one of my previous lives.
Back in the 1980's I was managing a rural ambulance station, a role which included manning an emergency ambulance as part of a crew of two, so I spent a lot of time with a regular crew mate who was a fundamentalist Creationist 7th Day Adventist.
We rarely spoke about religion (in the UK religion is a private matter and not a suitable subject for discussion in the workplace and anyway, he regarded me as a lost cause, Satan having blinded me with science) but somehow we had got onto the subject of everything in nature being designed by God, and to illustrate his point, my crewmate pointed to a small holly tree that was growing near the front entrance to the station and said its growth pattern was set by God so it would grow according to his plan ("Look at the trees!" Yes, that was the level of his theology). It was Spring and the tree was in active growth.
So I asked if I would be able to change God's plan by changing the growth pattern of the tree. "Of course not! That would make you more powerful than God, and nothing can be!"
In an even earlier life, I had been a senior medical research technician and had studied plant physiology, amongst other sciences, as part of my degree course, so I knew a little bit about plant hormones called gibberellins and auxins.
So, I pointed out that if you look at a growing shoot, you will notice that the side shoots get longer the further away they are from the growing tip. I then snipped off the tip of the growing shoot with a pair of scissors and told him to watch what happened over the next few days. After a few days it was obvious that side-shoots were now forming and growing quickly near what had been the growing tip with the longer shoots now closer to the tip in the reverse of what was the normal pattern of growth. I had changed his god's plan, apparently and reversed the growth pattern of a holly tree.
Over the next few weeks, I saw my crew mate frequently examining the holly tree and snipping off growing shoot himself.
Eventually, he conceded that I (and now he) could change the growth pattern of a holly tree, and asked my how it had happened. I explained that the cells in the growing tip (the meristem cells that were differentiating into the different cell types as the shoot grew) produced a type of hormone called auxin which inhibited the side shoots closer to the tip but its effects were diluted lower down the shoot and eventually, in low concentration, they were growth stimulants. By clipping off the tip, we had removed the hormone, so the previously inhibited side shoots were now free to grow. The growth pattern is not controlled by his god but by physiological processes.
Eventually, my crew mate concluded that his god had devised the hormone system so we weren't playing God and tinkering with his plan for the holly tree after all. God was safely back in the growing tips of the holly tree where he belonged, according to 7th Day Adventists.
You can lead a horse to water…
That was by way of an introduction to the work of scientists from Bristol University, UK, who have discovered how the earliest vascular plants evolved the ability to branch - a key development in the evolution of multicellular plants. What they have discovered is fundamental to distantly-related plants, strongly indicating common ancestry.
To make matters worse for Creationists, it all happened over 400 million years before they believe the Universe existed!
Photograph of S. kraussiana, showing its dichotomising shoot system
Light micrograph showing major and minor branches. Boxes in B indicate apices dissected and magnified in C and D.
Light micrograph showing major and minor shoot apices. Asterisks show the position of dissected leaf scars.
Scanning electron micrograph showing major and minor shoot apices with apical cells. Asterisks highlight dissected leaf scars.
Image showing successive dichotomies (D; white circles) and strong apical dominance in the S. kraussiana shoot.
(F-H) Light micrographs of a dorsal angle meristem (F), an emerging rhizophore (G) and an emerging angle shoot (H) at a dichotomy.
(F-H) Light micrographs of a dorsal angle meristem (F), an emerging rhizophore (G) and an emerging angle shoot (H) at a dichotomy.
(F-H) Light micrographs of a dorsal angle meristem (F), an emerging rhizophore (G) and an emerging angle shoot (H) at a dichotomy.
By studying the mechanisms responsible for branching, the team have determined what the first land plants are likely to have looked like millions of years ago.
Despite fundamentally different patterns in growth, their research has identified a common mechanism for branching in vascular plants.
As an ancient vascular plant lineage that formed coal seams during the Carboniferous era, lycophytes preserve the ancestral pattern of dichotomous branching.Diverse shapes abound in the dominant flowering plant group, and gardeners will be familiar with ‘pinching out’ plants’ shoot tips to stimulate side branch growth, leading to a bushier overall form. However, unlike flowering plants, other vascular plants branch by splitting the shoot apex into two during growth, a process known as ‘dichotomy’.
The greening of the land by plants paved the way for all terrestrial life to evolve as it provided food for animals and oxygen to breathe, and branching was a key innovation in the radiation of land plants.
Our work implies that branching evolved earlier than thought, which is an important evolutionary conclusion.
Aside from that, the fact that we have shown that plants that are so distantly related use the same genetic mechanisms to regulate branching brings great potential to transfer knowledge in engineering plant shape to improve future productivity and yield.
Dr Jill Harrison, corresponding author
School of Biological Sciences
University of Bristol, Bristol, UK.
Using surgical experiments in a lycophyte, researchers at the University of Bristol have discovered that dichotomy is regulated by short range auxin transport and co-ordinated in different parts of the plant by long range auxin transport.
Published in Development, the findings that both flowering plant and lycophyte branching are regulated by auxin transport imply that similar mechanisms were present in the earliest vascular plants around 420 million years ago.
By combining these findings with discoveries made in the non-vascular, non-branching moss group we can infer what the first land plants looked like around 480 million years ago.
Previously, the Dr Harrison’s lab disrupted auxin transport in a moss, leading it to branch in a similar manner to the earliest branching fossils.
Together these studies imply that the earliest land plants were branched, and that branching was lost during the evolution of non-vascular mosses.
Figure 6
S. kraussiana PINR and PINS are expressed in shoot apices and stem vasculature.
(A) Scanning electron micrographs of shoot apices undergoing anisotomy. The apical cells duplicate during dichotomy, and then separate to form new major (Ma) and minor (Mi) branch apices (white arrowheads). Angle meristems (AM, grey arrowhead) are evident shortly after dichotomy. Asterisks show initiating leaves. (B) Light micrographs of PIN in situ hybridisations during dichotomy. Dashed lines show the edge of the apex, and the major (Ma) and minor (Mi) branches are evident based on their relative size. In sagittal sections, the merophytes and the apical cell are outlined with dashed lines. Arrowheads indicate shoot apices; asterisks indicate initiating leaves; orange outlines indicate developing ligules. Va, vasculature. (C) Schematic summarising the expression patterns shown in B. PINR (blue) had broadly apical expression at all stages of dichotomy but was expressed less strongly in the initial cells (yellow) and merophytes (dashed area). PINS (purple) had similar expression to PINR but expression was more closely associated with the vasculature. PINT (green) had no detectable apical expression but was expressed in vascular development. Asterisks indicate developing leaves; black arrowheads indicate shoot apices. Sense controls are shown in Fig. S11. Scale bars: 0.02 mm.
S. kraussiana PINR and PINS are expressed in shoot apices and stem vasculature.
(A) Scanning electron micrographs of shoot apices undergoing anisotomy. The apical cells duplicate during dichotomy, and then separate to form new major (Ma) and minor (Mi) branch apices (white arrowheads). Angle meristems (AM, grey arrowhead) are evident shortly after dichotomy. Asterisks show initiating leaves. (B) Light micrographs of PIN in situ hybridisations during dichotomy. Dashed lines show the edge of the apex, and the major (Ma) and minor (Mi) branches are evident based on their relative size. In sagittal sections, the merophytes and the apical cell are outlined with dashed lines. Arrowheads indicate shoot apices; asterisks indicate initiating leaves; orange outlines indicate developing ligules. Va, vasculature. (C) Schematic summarising the expression patterns shown in B. PINR (blue) had broadly apical expression at all stages of dichotomy but was expressed less strongly in the initial cells (yellow) and merophytes (dashed area). PINS (purple) had similar expression to PINR but expression was more closely associated with the vasculature. PINT (green) had no detectable apical expression but was expressed in vascular development. Asterisks indicate developing leaves; black arrowheads indicate shoot apices. Sense controls are shown in Fig. S11. Scale bars: 0.02 mm.
More technical detail, including several stunning electron micrographs such as the above, are available in the team's open access paper in the journal Development:
ABSTRACTI wonder how Creationists will dismiss this evidence of life evolving on a very old Earth and of the shape of trees, which they traditionally point at as evidence of their putative designer god, which is determined by how growing shoots branch, being under the control of hormones and ultimately of genes that are evidence of common ancestry?
Diverse branching forms have evolved multiple times across the tree of life to facilitate resource acquisition and exchange with the environment. In the vascular plant group, the ancestral pattern of branching involves dichotomy of a parent shoot apex to form two new daughter apices. The molecular basis of axillary branching in Arabidopsis is well understood, but few regulators of dichotomous branching are known. Through analyses of dichotomous branching in the lycophyte, Selaginella kraussiana, we identify PIN-mediated auxin transport as an ancestral branch regulator of vascular plants. We show that short-range auxin transport out of the apices promotes dichotomy and that branch dominance is globally coordinated by long-range auxin transport. Uniquely in Selaginella, angle meristems initiate at each dichotomy, and these can develop into rhizophores or branching angle shoots. We show that long-range auxin transport and a transitory drop in PIN expression are involved in angle shoot development. We conclude that PIN-mediated auxin transport is an ancestral mechanism for vascular plant branching that was independently recruited into Selaginella angle shoot development and seed plant axillary branching during evolution.
Spencer, Victoria M. R.; Bentall, Lucy; Harrison, C. Jill (2023)
Diverse branching forms regulated by a core auxin transport mechanism in plants
Development 150(6), dev201209. DOI: 10.1242/dev.201209
Copyright: © 2023 The authors.
Published by The Company of Biologists. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
You can be absolutely certain they'll need to dismiss much of the evidence and misrepresent the rest of it, if they ever address the problem a paper like this causes them. What they will never do is wonder if this evidence means they could have got it all wrong.
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