If you fall down, your bones will break. Why didn't our ancestors create humans with a different material?

If we were to make a list of human obsessions for the future, mechanical metal skeletons would have to be on the list. From Iron Man in the Marvel series to the Major with a "mechanical prosthesis" in "Ghost in the Shell", they replaced the existing calcium skeletons of humans with steel alloys to become super humans.

As someone who once fell from a bicycle parabola and broke seven ribs, I can't help but ask: With so many elements in nature, why do we have to use calcium to make bones? Wouldn't gold, silver, silicon and nickel be better? If gold was the main component of bones, falling would be a piece of cake.

If you don't break any bones when you fall, you can just fall.

"Did you fall today?" "Yes, don't cue me."

Why did humans choose calcium bones? We have to go back hundreds of millions of years and start from the beginning of the story.

Bone Rise: What happened to the agreement that we would get soft together?

In the long evolution of billions of years, we still cannot give a definite answer to when life first had bones. But at least in the Ediacaran period between 635 million and 541 million years ago, only a very small number of organisms had bones, and most of the others were still soft. They lay on the seabed, not good at moving, feeding on microbial mats, and did not bother each other.

Reconstruction of the Ediacaran biota | Ryan Somma

Such a peaceful life was broken with the arrival of the Cambrian period. At this time, the earth warmed up, the number of plants on the seafloor increased, and a large amount of oxygen accumulated. This provided the earth with more food and a more suitable environment, giving rise to larger, faster-moving, and more oxygen-consuming animals. The surge in the types and forms of life brought about the so-called "Cambrian Explosion of Life".

What followed was increasingly fierce competition between species. Competitions between hunters and prey appeared in large numbers, and both attackers and defenders had to step up their "equipment upgrades." For example, they needed more advanced visual systems to accurately locate each other. The whole body was limp, and it was also impossible to meet the needs of rapid action.

Some organisms developed the ability to dig holes to hide at this time, such as the extinct Canadaspis, which allowed them to develop living space downwards | Claire H. / Wikimedia

They must prop themselves up and use hard structures to support their bodies. In this way, in order to eat and avoid being eaten, bones have become an inevitable part of the evolutionary process.

Making Bones: Who is the "Chosen One"?

Bone evolution is a long process, but we might as well condense thousands and tens of thousands of years of history into a small insect on the bottom of the Cambrian sea - let's forcibly name it "Tony". Whether as a hunter or a captured party, if Tony wants to survive in the Shura field of the Cambrian period, he must make himself hard. One of his ways out is biomineralization - depositing minerals to create a hard body structure.

Biomineralization has occurred long ago, but it was in full swing during the Cambrian period. At this time, the temperature rose, the glaciers melted, and the sea eroded the sedimentary rocks on the seashore, bringing metal ions such as calcium, iron, and potassium in the rocks into the seawater, which provided Tony and others with rich mineral raw materials.

But Tony still has many choices when it comes to which type of minerals to use.

Option 1: Sand fragments and plant debris

If Tony is not too demanding, he can use secretions to stick sand fragments (such as mica) and plant debris to the body. Although the technique is improvisation and the craftsmanship is crude, it can still be regarded as an early exoskeleton (shell) shape.

In the early Cambrian period, Onuphionella durhami, a small insect that glued mica sheets together as a "shell" | Signor & McMenamin

Option 2: Iron compounds

If Tony is a really tough little bug, he can choose an iron exoskeleton. For example, the horned snail not only has iron sulfide scales on its feet, but also has three layers of shells on its body, from the outside to the inside, namely iron sulfide shell, organic cuticle and calcium shell.

Must-have image of Chrysomallon squamiferum | Kentaro Nakamura et al.

Option 3: Silica

Tony could also become a first-generation silicon-based life form, using silicon dioxide to build an exoskeleton, like radiolarians.

Radiolarian fossils | Shan Chang, Qinglai Feng, Lei Zhang

Option 4: Calcium compounds

However, the examples mentioned above are rare, and the most mainstream choice is calcium compounds. Because for ordinary little bugs like Tony, when choosing minerals, it needs to consider many practical factors:

First, the content of this element in the environment must be high enough; second, organisms must be able to transport, regulate and utilize them at the cellular level in order to produce biominerals.

In the marine environment at the beginning of skeleton evolution, calcium ions were abundant, and almost all cells can regulate the calcium level in the body. Calcite and aragonite, the most common calcium carbonate minerals in nature, have become the main source of biomineralization and are the first choice for Tony and others to make exoskeletons.

So far, we can answer that at the beginning of evolution, our insect ancestors tried to use various minerals to build exoskeletons. But it may be the marine environment at that time and the mechanisms in most organisms that made calcium skeletons the biggest winner.

Small fossil shells from the early Cambrian period were the heavy armor (exoskeletons) of Tony | Precambrian Research

Biomineralization is a complex process involving the precipitation and growth of minerals, and requires the participation of various proteins. In the process of evolution, it is very difficult to change minerals. It is like a large factory suddenly changing to make other products. All related equipment must be adjusted and replaced, which is too costly.

By now, bone formation has been perfectly adapted to the environment, and there is no reason to change it. Calcium bones not only became the king when they debuted, but also became the "chosen one" that has been thriving for a long time.

Bone revelation: Hardness is the way to success

Life's perception of light induced the emergence and evolution of vision, which was like the first domino to fall, triggering the Cambrian explosion of life. The emergence of bones was like the transformation of the Hulk, rising from the ground and supporting a strong breakthrough on the road of evolution. These two are the results of evolution, and in turn have become the driving force of evolution, prompting the emergence of more species.

We are getting further and further down the path of skeletonization. Today, insects such as locusts and cockroaches, crustaceans such as crabs and shrimps, gastropods such as snails, and bivalves such as clams all have shells as their exoskeletons.

The evolution of the endoskeleton also occurred at almost the same time, and the vertebrate branch emerged. With the support of the spine, large vertebrates began their journey to land. After leaving the sea, for hundreds of millions of years, life explored, cooperated, chased and strangled each other on land, and after one mass extinction after another, humans with upright spines finally evolved.

In 2014, paleontologists discovered 550 million-year-old trace fossils on Newfoundland. The soft and immobile Ediacarans left irregular traces of their movements on the surface. We seem to see them stumbling and groping because they have no hard shell to support them. Because their nervous system has not evolved, they cannot perceive the surrounding environment, nor can they say hello to their neighbors.

Fossils from the Ediacaran period. Since only a few creatures had exoskeletons during this period, it was almost impossible for other soft animals to be preserved, so the existing fossils are mainly trace fossils, which record the remains or relics left behind when ancient creatures were active | Calla Carbone and Gut M. Narbonne

1.2 meters above this layer of fossils, the traces become regular and densely arranged on the sedimentary rocks. Among them, there is a winding trace line that suddenly becomes straight after a certain node.

Paleontologists speculate that this may be a trace of activity left by a small insect in the Cambrian period. It may have suddenly discovered a predator and quickly fled; or it may have been dragged away by the predator. This straight trace is probably left because of the hard bones. In the repeated strangulation, self-upgrade, and efforts to break through, this is the long journey of life in the history of evolution.

Scratches of Cambrian arthropods | Calla Carbone and Gut M. Narbonne

No one can accurately calculate how long this 1.2 meters corresponds to; it is difficult to piece together the full picture of skeletal evolution based on fossil evidence alone. But we can cautiously say that skeletal life appeared in large numbers within this 1.2 meters and continued to multiply - and one of their descendants just typed this line of words.

Think about it, 540 million years ago, a small insect faced a powerful opponent and built a skeleton for itself, letting fate submit to it; today, we continue to look forward to the emergence of mechanical skeletons in the science fiction world, eager to use them to build steel frames to support the hull in the huge waves and have control over our destiny.

Mechanical skeletons have become a common setting in science fiction works | "Ghost in the Shell"

The distant past and the unpredictable future seem to overlap here.

References

[1] Ben Yang, Michael Steiner b, Maoyan Zhu c, Guoxiang Li c, Jianni Liu d, Pengju Liu. 2016. Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution. Precambrian Research, 285: 202-215

[2] Carbone, C and Narbonne, MG 2014. When Life got Smart: the evolution of behavioral complexity through the Ediacaran and early Cambrian of NW Canada. Journal of Paleontology, 88(2): 309-330.

[3] Chang S, Feng Q, Zhang L. 2018. New Siliceous Microfossils from the Terreneuvian Yanjiahe Formation, South China: The Possible Earliest Radiolarian Fossil Record. Journal of Earth Science, 29(4): 912-919.

[4] Dzik.J. 2007. The Verdun Syndrome: simultaneous origin of protective armor and infaunal shelters at the Precambrian–Cambrian transition in Vickers, RP & Komarower, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 405–414.

[5] Fox, D. 2016. What sparked the Cambrian Explosion? Nature, 530: 268-270.

[6] Porter, MS 2007. Seawater Chemistry and Early Carbonate Biomineralization. Science, 316 (5829): 13.

[7] Signor, PW & McMenamin, MAS 1988. The Early Cambrian worm tube Onuphionella from California and Nevada. Journal of Paleontology, 62: 233-240.

[8] Watson, T. 2020. These bizarre ancient species are rewriting animal evolution. Nature, [online] Volume 586, p. 662-665. Available at:

https://www.nature.com/articles/d41586-020-02985-z [Accessed 15 Jan. 2021].

[9] Wikipedia. 2020. “Cambrian.” Last modified September 12, 2020.

https://en.wikipedia.org/wiki/Cambrian.

Author: Xiaodao

Editor: Mai Mai, Rain Knocking on the Window