June 15, 2022 | David F. Coppedge

How Organisms Know When to Stop Growing

A common observation becomes
a mystery when examined in detail.


Many families put up a strip of paper by a door or wall to let the children see how fast they are growing. Progress is recorded on birthdays by marking heights on the strip, or by taking photographs. That little newborn rapidly increases in size till it is weaned, and continues its rise to adult size over about 18 years. Everyone knows to expect the adolescent growth spurt. Preteens and teen-agers compare themselves at school, the short ones worrying and the tall ones feeling self conscious. But by adulthood, barring genetic issues, most have reached a normal distribution of height for their ethnic group, with most in the middle range and a few shorties and giants on the extremes.

Why do we all stop growing? Why doesn’t the growth spurt continue at the same rate indefinitely? Scientists like to ask such questions that laypersons take for granted. When you think of it, every species of plant and animal has its normal adult size – hundreds of feet high for a coast redwood, three inches for some butterflies, a foot for the pet tabby cat (not counting the tail), a few inches for a petunia plant, and so forth. Even microbes stop growing at some point, and the mighty Ultrasaurus never reached the stratosphere.

At the cellular and tissue level, DNA doesn’t have eyes or rulers to monitor the overall growth of the organism. Something keeps track of growth and tells the organism to stop growing when the normal size is reached. What is it? Two new studies agree this is a profound biological mystery, and barely scratch the surface trying to explain it.


Adult animals encompass an astounding range of sizes.

How animals reach their correct size (Friedrich Miescher Institute, 7 June 2022).

Benjamin Towbin, an athletic young professor who studied in Israel and Germany, embarked on a project to try to figure out an “astounding” phenomenon: how do animals reach their correct size?

By and large, individuals of the same species grow to the same size. This uniformity in size is astounding, since intrinsic randomness in developmental processes and in environmental conditions produce substantial differences in how fast individuals grow. Moreover, because animal growth is often exponential, even small differences in growth can amplify to large differences in size. How do animals nevertheless make sure to reach the correct size?

Towbin and his colleagues used the common lab roundworm Caenorhabditis elegans. He watched hundreds of the tiny roundworms grow. Knowing that developing worms do not grow at the same rate, and that a 10% difference in initial growth rate could result in a 100% size difference, he wondered why most of the worms ended up within the normal size distribution for this species. From his observations, he hypothesized that a “genetic oscillator” is responsible.

The mechanism does not appear to measure size per se. Instead, it senses how fast an individual grows and appropriately adjusts the time after which this individual turns adult. Therefore, a slowly growing individual reaches the same size as a rapidly growing individual because it is given more time to grow.

The team published their results and conclusions in Nature Communications on 6 June 2022 (open access). But surely this hypothesis only scratches the surface of the opening question about how animals reach their correct size. He only observed tiny invertebrate worms. What happens in giraffes, salamanders, gorillas, koalas, octopus, mice and sauropods? Towbin apparently had little use for evolutionary theory. Except for a passing reference that “growth to the appropriate size is indeed under strong selective pressure,” the paper finds better explanatory power in a design principle:

Transcriptional oscillations are found in numerous organisms. Most famously, circadian clocks control oscillations that match the diurnal cycle. Unlike the developmental clock of C. elegans, the 24 h period of circadian clocks is robust to fluctuations in growth rates or temperature. We propose that the apparent lack of robustness of developmental oscillations to changes in growth rates in return provides robustness to body size. It will be interesting to see if this design principle also applies to the size homeostasis of other multicellular systems, such as the somitogenesis clock in vertebrates.


Giant Kauri tree in New Zealand (DFC)

How do plants know how big to grow? (Carnegie Science, 7 June 2022).

At about the same time as the previous paper, a publication on plant growth looked into a similar question. Researchers at Palo Alto, California, knowing that “Organisms grow to fit the space and resources available in their environments, leading to a vast diversity of body sizes and shapes within a population of the same species,” pondered the same question for plants: “What are the genetic and physiological mechanisms that determine how big an organism can grow?” For plants, the mystery has “puzzled scientists for generations.”

Investigating the common lab plant Arabidopsis thaliana, this team discovered a gene they believe is responsible. They named it CHIQUITA1 and found that it’s part of a family of genes that govern plant size. This gene acts like a switch to stop the proliferation of cells, so that they differentiate into tissues and organs. They published their results in the journal Development with the title, “CHIQUITA1 maintains the temporal transition between proliferation and differentiation in Arabidopsis thaliana.”

Again, though, this study involved one species of herbaceous plant. What about pine trees, kelp forests, fungi, liverworts, wildflowers, crop vegetables, and fruit trees? They can say that “Our work reveals the interdependency between cellular and organ-level processes underlying final organ size determination,” but they have only scratched the surface. This team, too, apparently did not need to appeal to Darwinian theory. Instead, they were looking for rules that govern processes. “Unlike most research on cell proliferation, this work was done on the individual cell level, rather than at the population level, highlighting how much more there is to learn about the rules that govern plant biology.” That sounds a lot like engineering.

Runaway growth would quickly outstrip resources available for an organism, but starvation itself is not a “mechanism” for limiting growth. Intelligent designers such as engineers have the foresight to create processes and rules that lead to a desired outcome. When we see “design principles” at work in biology, we can infer that a designing intelligence was involved in the origin of regulated developmental processes that can take a zygote to a worm, a small plant, or a whale or towering cedar. That makes sense. Darwinist “magical thinking” (see Neil Thomas at Evolution News) is a type of philosophical non-sense, “in the philosophical sense of not having sufficient logical stringency to merit serious discussion.” Stuff Happens is no answer at all.


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Categories: Amazing Facts, Biology

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