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In this week’s newsletter

✅ The Modular Human. ✅ A tale of two longbio companies. ✅ The AI doctor should see you now. ✅ XPRIZE Healthspan has named its semifinalists. ✅ How to get billionaires to invest in longevity.

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The Modular Human: How to solve aging with replacement parts

The replacement strategy is the idea that, to solve aging, we might not need to repair every damaged component - or even fully understand how the damage happened. Instead, we could simply replace the worn parts.

But depending on who you talk to, this strategy can sound either wildly ambitious or surprisingly practical. At one end of the spectrum, it conjures visions of cloned, brainless bodies* and piecemeal brain transplants. At the other, it's a logical extension of surgical medicine - anchored in decades of experience with hip replacements, pacemakers, and stem-cell therapies.

* Something that I personally have absolutely no qualms about. But many do.

A new Nature Aging paper ”Replacement as an Aging Intervention”, led by Sierra Lore* and co-authored by George Church, Eric Verdin, Vadim Gladyshev, Anthony Atala and others - lands firmly in the latter camp.

* I met Sierra at the Longevity Biotech Fellowship retreat in Sweden back in 2024.

The paper introduces replacement interventions as a coherent class of anti-aging therapies: treatments that restore or substitute aged cells, organs, or even brain systems. It’s billed as the first unified framework to treat these diverse technologies not as isolated last-resort procedures, but as a preventive, strategic response to aging itself.

What it boils down to, essentially, is assembling tools we already have - some FDA-approved, some in early trials - into a long-term, organ-by-organ anti-aging roadmap.

So, what’s on the menu?

The paper divides replacement interventions into two major groups:

Biological replacements: living tissues and organs derived from stem cells, xenografted from pigs, or built using organoid/bioprinting approaches.

Synthetic replacements: pacemakers, neural prostheses, brain-machine interfaces.

Each group is further broken down by scale - cellular, organ, and brain - illustrating just how far replacement technologies have already come.

Replacement is presented here as a mature, modular, and scalable strategy. In Sierra Lore’s words, “We already have the tools to rebuild and rejuvenate the aging body.”

But what about the more radical vision?

If you’ve followed the Longevity Biotech Fellowship’s roadmap, or read Jean Hebert’s Replacing Aging, you’ll know that the replacement conversation doesn’t stop at xenotransplants and brain implants.

LBF’s “Replacement” section - part of their broader technical plan to solve aging - embraces a far more ambitious model:

• Whole cloned body replacement, using non-sentient bodies grown without functional neocortex (à la hydrancephaly).

• Progressive brain replacement, where cortical areas are silenced and replaced incrementally to preserve identity.

They estimate a full proof-of-concept program (body + brain + spinal reconnection) would cost around $3.6B and could be achievable within 10 years.

The Nature paper doesn’t go there. It avoids the ethically thornier ground of brainless clones and neural transfer into synthetic or biological chassis. It also sidesteps a potentially fundamental limitation: whether memories not actively recalled during progressive brain replacement could be lost for good.*

* This was something Aubrey de Grey - not the biggest replacement supporter - touched upon when we had him on the podcast recently.

So while both visions share a conceptual root - replacement over repair - the tone and ambition are quite different.

But still, it’s somewhat of a milestone moment. The fact that a journal like Nature Aging is now legitimizing this paradigm is itself a breakthrough.

For years, the dominant frameworks in longevity science were shaped by either repair-oriented models or pharmacological modulation. Now, replacement is being reframed as the most immediately actionable, technically mature, and scalable approach we have.

News from around the longevity and health space.

A tale of two longbio companies

It was the best of times, it was the worst of times: Here’s a recent tale of two longbio companies.

First, Unity Biotechnology. I remember reading about Unity nearly a decade ago, when senolytics were the hottest idea in aging. Their co-founder Nathaniel David was a proto–Bryan Johnson figure, described in The New Yorker as having “an unlined face and thick auburn hair,” looking 30 at 49. “We think our drugs vaporize a third of human diseases in the developed world,” he said.

Fast forward eight years - and the picture couldn’t be more different.

This month, Unity released full results from its Phase 2b ASPIRE trial and announced it is laying off its entire staff, cutting costs, and exploring “strategic alternatives” - biotech shorthand for asset sales, licensing, merger, or shutdown.

The company’s lead therapy, UBX1325, a senolytic for diabetic macular edema, missed its primary endpoint. In an interview with Longevity.Technology CEO Anirvan Ghosh framed it as a narrow miss: “Having run a lot of trials, when a study doesn’t work, it’s rarely just one small measure falling short while everything else looks good. But that’s exactly what happened in our case.”

To be fair, Unity did help move senescence-targeting drugs from theory to human trials. As Ghosh put it, their data “shows that targeting senescent cells can lead to improved outcomes.” But in the end, progress wasn’t enough.

Meanwhile, the spotlight has shifted.

On May 6, NewLimit, co-founded by Coinbase CEO Brian Armstrong, announced a $130 million Series B led by Kleiner Perkins.

NewLimit’s pitch? Epigenetic reprogramming is the key. Reprogramming now sits at the center of the aging field. NewLimit claims to run the largest reprogramming experiments in the world, using the resulting data to train AI models that prioritize which drug candidates to test.

The company says the therapeutic opportunity here is “100X larger than any biotech product that has come before.”

XPRIZE Healthspan has named its semifinalists

XPRIZE has named 100 semifinalists (from over 600 entrants) for its $101M Healthspan Prize, with 40 teams awarded early funding. The entries span a wide range: mitochondrial repair, immune rejuvenation, exosomes, and repurposed drugs like GLP-1s. While there are bold outliers - like Unlimited Bio’s offshore gene therapy cocktail - the majority of teams pursue relatively safe, near-term strategies. Notably absent is many partial reprogramming efforts.

The field is unusually diverse, including everything from underfunded two-person teams to well-capitalized companies like BioAge Labs. Other standouts include Intervene Immune, Ani Biome, and Kimera Labs. You can learn more about all semifinalists here.

The AI doctor should see you now

In a previous LEVITY post, I argued that AI - and eventually humanoid robots - could outperform human doctors across much of medicine. Well, it’s starting to look like a very safe bet indeed.

OpenAI just launched HealthBench, a massive new benchmark based on 5,000 realistic health dialogues and 48,000 physician-crafted evaluation criteria. Their latest models now outperform human clinicians in many dimensions - including core capabilities such as instruction-following and accuracy, as well as in complex scenarios like global health.

Meanwhile, Google’s newly updated medical chatbot AMIE was tested head-to-head against primary care physicians in simulated consultations involving real images, ECGs, and lab reports. In terms of diagnostic accuracy? AMIE won.

These systems don’t just memorize symptoms - they reason, contextualize, and even have better bedside manner.

My argument stands: it won’t be long before human doctors who don’t consult AI will be considered negligent. And in the long run we will come to understand that AI alone can do the better job. Consider this your second opinion.

Worth your time.

How to get billionaires to invest in longevity

The longevity tech tree is unlocked. But almost nobody has realized it. That’s the take-home message from VC firm NFX’ call to action for investors and the public alike. We now have the scientific tools to tackle aging itself, but a massive funding gap is holding progress back. This piece reframes longevity not as sci-fi or vanity, but as both a moral imperative and a trillion-dollar opportunity. Everyone should read it.

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